Solar Minimum Conditions Research Articles

From solar to stellar flare characteristics

Context. The connection between solar energetic proton events and X-ray flares has been the focus of many studies over the past 13 yr. In the course of these investigations several peak size distribution functions based on Geostationary Operational Environmental Satellite (GOES) measurements of both quantities have been developed. In more recent studies one of these functions has been used to estimate the stellar proton fluence around the M-dwarf star AD Leonis. However, a comparison of the existing peak size distribution functions reveals strong discrepancies with respect to each other. Aims. The aim of this paper is to derive a new peak size distribution function that can be utilized to give a more realistic estimate of the stellar proton flux of G-, K-, and M-dwarf stars. Methods. By updating and extending the GOES-based peak size distribution down to B-class X-ray flare intensities with the help of SphinX data from the solar minimum conditions of 2009 and newly derived GOES data between 1975 and 2005, we developed a new power-law peak size distribution function for solar proton fluxes (E > 10 MeV). However, its resulting slope differs from values reported in the literature. Therefore, we also developed a double-power-law peak size distribution function. An extension to much higher X-ray flare intensities (10−1) W m−2 and above, for the first time, results in an approximation of best- and worst-case scenarios of the stellar proton flux around G-, K-, and M-dwarf stars. Results. Investigating the impact of the newly developed peak size distribution function for G-, K-, and M-dwarf star flare intensities we show that in the worst-case scenario previous studies may underestimate the stellar proton flux by roughly one to five orders of magnitude.

NON-TARGETED EFFECTS LEAD TO A PARIDIGM SHIFT IN RISK ASSESSMENT FOR A MISSION TO THE EARTH'S MOON OR MARTIAN MOON PHOBOS.

Cancer risk is an important limitation for galactic cosmic ray (GCR) exposures, which consist of a wide-energy range of protons, heavy ions and secondary radiation produced in shielding and tissues. Many studies suggest non-targeted effects (NTEs) occur for low doses of high-linear energy transfer (LET) radiation, leading to deviation from the linear dose response model used in radiation protection. We investigate corrections to quality factors (QF) for NTEs, which are used in predictions of fatal cancer risks for exploration missions. Prediction of fatal cancer risks for missions to the Martian moon, Phobos of 500-d and the Earth's moon of 365-d for average solar minimum condition show increases of 2- to 4-fold higher in the NTE model compared with the conventional model. Limitations in estimating uncertainties in NTE model parameters due to sparse radiobiology data at low doses are discussed.

Measurement of plasma channels in the Venus wake

Plasma channels have been suggested to account for the observation of ionospheric holes in the Venus nightside ionosphere. They are produced by the erosion of the solar wind on the magnetic polar regions of the Venus ionosphere, and are related to a sharp plasma transition that extends along the flanks of the wake and that is located downstream from the bow shock. The sharp plasma (intermediate) transition is a feature that has been detected with instruments onboard all the main spacecraft that have probed the Venus wake. From the early flyby transit of the Mariner 5 spacecraft to the later orbit crossings of the Venera 10, the Pioneer Venus and more recently the Venus Express there has been accumulated evidence of the presence of that transition by the flanks of the Venus wake. We report here measurements made with the ASPERA-4 instrument of the Venus Express that were conducted in orbit crossings by the midnight plane over a 4 year period (between 2006 and 2009), and that show different positions of the intermediate transition in the Venus wake at locations closer to the sun-Venus axis near solar minimum conditions by 2009. Measurements conducted in this latter time period also indicate that the intermediate transition by the midnight plane derives from the plasma channels and that is replaced by a broad velocity layer in orbits that probed farther away from that plane. It is argued that changes in the position of the intermediate transition with respect to the plasma channels are related to processes produced by the transport of solar wind momentum to the Venus polar ionosphere. The energy spectra of planetary ions measured in VEX orbits that probed by the midnight plane show momentum transport from the solar wind protons. This result is contrary to the conclusions reported by Collinson et al. (2014) in the sense that there is ¨no difference in the peak energy of the ionospheric outflow¨ when measured inside and outside an ionospheric hole.

Precipitation of Hydrogen Energetic Neutral Atoms at the Upper Atmosphere of Mars

AbstractWe study the properties of neutral hydrogen atoms precipitating onto the upper atmosphere of Mars. Energetic neutral atoms (ENAs) are produced by the charge exchange process between protons of solar wind (both upstream and shocked) as well as planetary origins and the Martian exospheric neutrals. Using a global hybrid plasma model for Mars‐solar wind interaction combined with an up‐to‐date exosphere model of Mars, we calculate the fluxes, spatial distributions, energy spectra, and direction distributions of hydrogen ENAs (H‐ENAs) at the Martian exobase for each source proton population. H‐ENAs originating from the upstream solar wind region and the magnetosheath dominate the precipitation. Two percent of the solar wind flux penetrates through the magnetic barrier as H‐ENAs under solar minimum conditions. The precipitating solar wind H‐ENA flux is axially symmetric about Sun‐Mars line, while the magnetosheath and planetary H‐ENAs have higher fluxes and a more‐extended precipitation area in the hemisphere where the convection electric field is pointing away from the planet, causing a significant precipitation beyond the terminator. The observed asymmetry is consistently explained by an asymmetric plasma flow in the dayside magnetosheath. The solar wind dynamic pressure increases the solar wind H‐ENA precipitation normalized by the upstream proton flux, due to a closer bow shock position and thus a higher exospheric column density for charge exchange. The spatial distribution of the magnetosheath solar wind and planetary H‐ENAs becomes more axially symmetric with increased dynamic pressure. The solar wind interaction with Mars exhibits more gas‐dynamic‐like signatures for higher dynamic pressure.

Cancer and circulatory disease risks for a human mission to Mars: Private mission considerations

In addition to traditional interest by various governments in space exploration, there is growing interest in private missions to Mars and other deep space destinations within the next decade. Private missions could consider persons not restricted by radiation limits; however there remains an interest in the level of risk to be encountered. The major risk for space travel is cancer from galactic cosmic rays (GCR), while circulatory diseases in suggested in some but not all epidemiology studies at modest doses (<1 Gy) and detriments in cognition are suggested by rodent studies following acute irradiation with moderate doses of heavy ions. The GCR are not easily shielded since they consist of high energy protons, heavy ions and secondary radiation produced in shielding and tissue. Furthermore heavy ions are more effective per unit dose in causing solid cancers compared to gamma-rays. In addition non-targeted effects (NTEs) are suggested by most low dose radiobiology studies to increase biological effectiveness for low doses of high LET radiation. Astronauts and cosmonauts are typically above 40-y, while younger aged persons could participate in private space missions. In this paper, we describe cancer and circulatory disease risks for a 940 d Mars mission for average solar minimum conditions for persons of varying ages from 20 to 60 years. For the first-time NTEs are considered in Mars mission cancer risk predictions. Cancer morbidity risks and 95% confidence intervals for age 20-y persons are predicted as 20.9% [7.04, 51.4] and 12.7% [4.97, 29.3] for females and males, respectively. We find that cancer fatality risks decline with age of exposure while circulatory disease risks are nearly independent of age of exposure. The ratio of cancer to circulatory disease fatalities decreases from about 8-to-1 at 20 y to 5-to-1 at 60 y in females and 4-to-1 and 2.5-to-1 in males with about 2-times higher loss of life expectancy for cancer deaths compared to circulatory related deaths, indicating the much higher importance of cancer risk compared to circulatory disease risks for persons participating in space missions.

Statistics of Durations and Spacings of Equatorial Plasma Depletions Detected by the C/NOFS Planar Langmuir Probe

AbstractThe Planar Langmuir Probe onboard the Communication/Navigation Outage Forecasting System satellite monitored ionospheric plasma densities and their irregularities with high resolution almost seamlessly for approximately five years since May 2008. Planar Langmuir Probe data from two different years (1 October 2008 to 30 September 2009 and 5 June 2012 to 4 June 2013) were selected for analysis. The first data set corresponds to solar minimum conditions, and the second one is as close to solar maximum conditions of solar cycle 24 as possible, with basis on their continuous availability. The results from a level‐crossing analysis applied to detrended evening‐time data from a fixed altitude range show how depletion durations and spacings between consecutive depletions are statistically distributed as functions of solar activity and longitude sector. In quantitative terms, they show that (i) the fewer depletions observed in the solar‐minimum year are also shorter, shallower, and closer together than those detected in the more active one and (ii) great similarities exist between structures of depletion durations and spacings between consecutive depletions observed in the South American and African longitude sectors, which are more noticeable than those associated with in the Indian‐Asian longitude sector.

On ozone trend detection: using coupled chemistry–climate simulations to investigate early signs of total column ozone recovery

Abstract. Total column ozone values from an ensemble of UM-UKCA model simulations are examined to investigate different definitions of progress on the road to ozone recovery. The impacts of modelled internal atmospheric variability are accounted for by applying a multiple linear regression model to modelled total column ozone values, and ozone trend analysis is performed on the resulting ozone residuals. Three definitions of recovery are investigated: (i) a slowed rate of decline and the date of minimum column ozone, (ii) the identification of significant positive trends and (iii) a return to historic values. A return to past thresholds is the last state to be achieved. Minimum column ozone values, averaged from 60° S to 60° N, occur between 1990 and 1995 for each ensemble member, driven in part by the solar minimum conditions during the 1990s. When natural cycles are accounted for, identification of the year of minimum ozone in the resulting ozone residuals is uncertain, with minimum values for each ensemble member occurring at different times between 1992 and 2000. As a result of this large variability, identification of the date of minimum ozone constitutes a poor measure of ozone recovery. Trends for the 2000–2017 period are positive at most latitudes and are statistically significant in the mid-latitudes in both hemispheres when natural cycles are accounted for. This significance results largely from the large sample size of the multi-member ensemble. Significant trends cannot be identified by 2017 at the highest latitudes, due to the large interannual variability in the data, nor in the tropics, due to the small trend magnitude, although it is projected that significant trends may be identified in these regions soon thereafter. While significant positive trends in total column ozone could be identified at all latitudes by ∼ 2030, column ozone values which are lower than the 1980 annual mean can occur in the mid-latitudes until ∼ 2050, and in the tropics and high latitudes deep into the second half of the 21st century.

The Morphology of the 6‐Day Wave in Both the Neutral Atmosphere and F Region Ionosphere Under Solar Minimum Conditions

AbstractCombined Thermosphere‐Ionosphere‐Mesosphere Energetics and Dynamics (TIMED)/TIMED Doppler interferometer horizontal winds, Global Positioning System Total Electron Content maps, and Constellation Observing System for Meteorology, Ionosphere, and Climate electron density profiles are utilized to study the westward zonal wavenumber 1 quasi‐6‐day planetary wave in both the neutral atmosphere and F region ionosphere under solar minimum conditions in 2009. It is found that the zonal (meridional) wind perturbations maximize (minimize) in the geographic equatorial mesosphere and lower thermosphere region and are in phase (out of phase) between hemispheres, which agrees well with both the Upper Atmosphere Research Satellite/high‐resolution Doppler imager wind observations and (1, 1) Rossby normal mode. Analysis of the Total Electron Content maps and Constellation Observing System for Meteorology, Ionosphere, and Climate electron density profiles shows that the F region ionosphere exhibits wave spectra and temporal variations that are consistent with those of the neutral atmosphere. Specifically, the 6‐day oscillation in electron density is the highest during 1200–1800 LT in the equatorial ionospheric anomaly region (~20° geomagnetic latitude), with a minimum amplitude at the equator, which agrees well with the F region fountain effect. At 300 km, the amplitude of the electron density oscillation reaches ~1.2 × 104 cm−3, which accounts for ~14% of the background value. In addition, the F2 layer electron peak density and peak height also exhibit corresponding oscillations with amplitudes of ~9 × 104 cm−3 and ~12 km, respectively. The current work provides clear evidence for F region ionospheric responses to planetary waves in the mesosphere and lower thermosphere region.

Weakened Magnetization and Onset of Large-scale Turbulence in the Young Solar Wind—Comparisons of Remote Sensing Observations with Simulation

Abstract Recent analysis of Solar-Terrestrial Relations Observatory (STEREO) imaging observations have described the early stages of the development of turbulence in the young solar wind in solar minimum conditions. Here we extend this analysis to a global magnetohydrodynamic (MHD) simulation of the corona and solar wind based on inner boundary conditions, either dipole or magnetogram type, that emulate solar minimum. The simulations have been calibrated using Ulysses and 1 au observations, and allow, within a well-understood context, a precise determination of the location of the Alfvén critical surfaces and the first plasma beta equals unity surfaces. The compatibility of the the STEREO observations and the simulations is revealed by direct comparisons. Computation of the radial evolution of second-order magnetic field structure functions in the simulations indicates a shift toward more isotropic conditions at scales of a few Gm, as seen in the STEREO observations in the range 40–60 R ⊙. We affirm that the isotropization occurs in the vicinity of the first beta unity surface. The interpretation based on early stages of in situ solar wind turbulence evolution is further elaborated, emphasizing the relationship of the observed length scales to the much smaller scales that eventually become the familiar turbulence inertial range cascade. We argue that the observed dynamics is the very early manifestation of large-scale in situ nonlinear couplings that drive turbulence and heating in the solar wind.

Fe/O ratio behavior as an indicator of solar plasma state at different solar activity manifestations and in periods of their absence

We report the results of the investigation into plasma physical characteristics at various solar activity manifestations and in periods of their absence. These results have been obtained from quantitative estimates of the relative abundance of Fe/O ions in different energy ranges. Maximum values of the Fe/O ratio is shown to correspond to particle fluxes from impulsive flares for ions with energies &lt;2 MeV/n (the most significant manifestation of the FIP effect). In particle fluxes from gradual flares, the Fe/O value decreases smoothly with ion energy and is noticeably inferior to values of fluxes in impulsive events. We have established that the properties of flares of solar cosmic rays indicate their belonging to a separate subclass in the total population of gradual events. Relying on variations in the abundance of Fe/O ions, we propose an xplanation of the solar plasma behavior during the development of flares of both classes. Magnetic clouds (a separate type of coronal mass ejections (CME)), which have regions of turbulent compression and are sources of strong geomagnetic storms, exhibit a relative composition of Fe ions comparable to the abundance of Fe in ion fluxes from gradual flares. We have found out that the Fe/O value can be used to detect penetration of energetic flare plasma into the CME body at the initial phase of their joint development and to estimate its relative contribution. During solar minimum with complete absence of sunspots, the Fe/O ratio during periods of “quiet” solar wind show absolutely low values of Fe/O=0.004–0.010 in the energy range from 2–5 to 30 MeV/n. This is associated with the manifestation of the cosmic ray anomalous component, which causes an increase in the intensity of ion fluxes with a high first ionization potential, including oxygen (O), and elements with a low first ionization potential (Fe) demonstrate weakening of the fluxes. As for particles with higher energies (Ek&gt;30 MeV/n), the Fe/O increase is due to the decisive influence of galactic cosmic rays on the composition of impurity elements in the solar wind under solar minimum conditions. The relative content of heavy elements in galactic cosmic rays 30–500 MeV/n is similar to values in fluxes from gradual flares during high solar activity. During solar minimum without sunspots, the behavior of Fe/O for different ion energy ranges in plasma flows from coronal holes (CH) and in the solar wind exhibits only minor deviations. At the same time, plasma flows associated with the disturbed frontal CH region can be sources of moderate geomagnetic storms.

Поведение отношения Fe/O как показателя состояния солнечной плазмы при различных проявлениях активности и в периоды ее отсутствия

We report the results of the investigation into plasma physical characteristics at various solar activity manifestations and in periods of their absence. These results have been obtained from quantitative estimates of the relative abundance of Fe/O ions in different energy ranges. Maximum values of the Fe/O ratio is shown to correspond to particle fluxes from impulsive flares for ions with energies &lt;2 MeV/n (the most significant manifestation of the FIP effect). In particle fluxes from gradual flares, the Fe/O value decreases smoothly with ion energy and is noticeably inferior to values of fluxes in impulsive events. We have established that the properties of flares of solar cosmic rays indicate their belonging to a separate subclass in the total population of gradual events. Relying on variations in the abundance of Fe/O ions, we propose an xplanation of the solar plasma behavior during the development of flares of both classes. Magnetic clouds (a separate type of coronal mass ejections (CME)), which have regions of turbulent compression and are sources of strong geomagnetic storms, exhibit a relative composition of Fe ions comparable to the abundance of Fe in ion fluxes from gradual flares. We have found out that the Fe/O value can be used to detect penetration of energetic flare plasma into the CME body at the initial phase of their joint development and to estimate its relative contribution. During solar minimum with complete absence of sunspots, the Fe/O ratio during periods of “quiet” solar wind show absolutely low values of Fe/O=0.004–0.010 in the energy range from 2–5 to 30 MeV/n. This is associated with the manifestation of the cosmic ray anomalous component, which causes an increase in the intensity of ion fluxes with a high first ionization potential, including oxygen (O), and elements with a low first ionization potential (Fe) demonstrate weakening of the fluxes. As for particles with higher energies (Ek&gt;30 MeV/n), the Fe/O increase is due to the decisive influence of galactic cosmic rays on the composition of impurity elements in the solar wind under solar minimum conditions. The relative content of heavy elements in galactic cosmic rays 30–500 MeV/n is similar to values in fluxes from gradual flares during high solar activity. During solar minimum without sunspots, the behavior of Fe/O for different ion energy ranges in plasma flows from coronal holes (CH) and in the solar wind exhibits only minor deviations. At the same time, plasma flows associated with the disturbed frontal CH region can be sources of moderate geomagnetic storms.

Neutralized solar wind ahead of the Earth's magnetopause as contribution to non-thermal exospheric hydrogen

Abstract. In a most recent paper by Qin and Waldrop (2016), it had been found that the scale height of hydrogen in the upper exosphere of the Earth, especially during solar minimum conditions, appears to be surprisingly large. This indicates that during minimum conditions when exobasic temperatures should be small, large exospheric H-scale heights predominate. They thus seem to indicate the presence of a non-thermal hydrogen component in the upper exosphere. In the following parts of the paper we shall investigate what fraction of such expected hot hydrogen atoms could have their origin from protons of the shocked solar wind ahead of the magnetopause converted into energetic neutral atoms (ENAs) via charge-exchange processes with normal atmospheric, i.e., exospheric hydrogen atoms that in the first step evaporate from the exobase into the magnetosheath plasma region. We shall show that, dependent on the sunward location of the magnetopause, the density of these types of non-thermal hydrogen atoms (H-ENAs) becomes progressively comparable with the density of exobasic hydrogen with increasing altitude. At low exobasic heights, however, their contribution is negligible. At the end of this paper, we finally study the question of whether the H-ENA population could even be understood as a self-consistency phenomenon of the H-ENA population, especially during solar activity minimum conditions, i.e., H-ENAs leaving the exosphere being replaced by H-ENAs injected into the exosphere. Keywords. Magnetospheric physics (plasmasphere; solar wind-magnetosphere interactions) – solar physics, astrophysics, and astronomy (energetic particles)

Development and Validation of the Whole Atmosphere Community Climate Model With Thermosphere and Ionosphere Extension (WACCM‐X 2.0)

AbstractKey developments have been made to the NCAR Whole Atmosphere Community Climate Model with thermosphere and ionosphere extension (WACCM‐X). Among them, the most important are the self‐consistent solution of global electrodynamics, and transport of O+ in the F‐region. Other ionosphere developments include time‐dependent solution of electron/ion temperatures, metastable O+ chemistry, and high‐cadence solar EUV capability. Additional developments of the thermospheric components are improvements to the momentum and energy equation solvers to account for variable mean molecular mass and specific heat, a new divergence damping scheme, and cooling by O(3P) fine structure. Simulations using this new version of WACCM‐X (2.0) have been carried out for solar maximum and minimum conditions. Thermospheric composition, density, and temperatures are in general agreement with measurements and empirical models, including the equatorial mass density anomaly and the midnight density maximum. The amplitudes and seasonal variations of atmospheric tides in the mesosphere and lower thermosphere are in good agreement with observations. Although global mean thermospheric densities are comparable with observations of the annual variation, they lack a clear semiannual variation. In the ionosphere, the low‐latitude E × B drifts agree well with observations in their magnitudes, local time dependence, seasonal, and solar activity variations. The prereversal enhancement in the equatorial region, which is associated with ionospheric irregularities, displays patterns of longitudinal and seasonal variation that are similar to observations. Ionospheric density from the model simulations reproduces the equatorial ionosphere anomaly structures and is in general agreement with observations. The model simulations also capture important ionospheric features during storms.

Prospective Out-of-ecliptic White-light Imaging of Coronal Mass Ejections Traveling through the Corona and Heliosphere

Abstract The in-flight performance of the Coriolis/SMEI and STEREO/HI instruments substantiates the high-technology readiness level of white-light (WL) imaging of coronal mass ejections (CMEs) in the inner heliosphere. The WL intensity of a propagating CME is jointly determined by its evolving mass distribution and the fixed Thomson-scattering geometry. From their in-ecliptic viewpoints, SMEI and HI, the only heliospheric imagers that have been flown to date, integrate the longitudinal dimension of CMEs. In this paper, using forward magnetohydrodynamic modeling, we synthesize the WL radiance pattern of a typical halo CME viewed from an out-of-ecliptic (OOE) vantage point. The major anatomical elements of the CME identified in WL imagery are a leading sheath and a trailing ejecta; the ejecta-driven sheath is the brightest feature of the CME. The sheath, a three-dimensional (3D) dome-like density structure, occupies a wide angular extent ahead of the ejecta itself. The 2D radiance pattern of the sheath depends critically on viewpoint. For a CME modeled under solar minimum conditions, the WL radiance pattern of the sheath is generally a quasi-straight band when viewed from an in-ecliptic viewpoint and a semicircular arc from an OOE viewpoint. The dependence of the radiance pattern of the ejecta-driven sheath on viewpoint is attributed to the bimodal nature of the 3D background solar wind flow. Our forward-modeling results suggest that OOE imaging in WL radiance can enable (1) a near-ecliptic CME to be continuously tracked from its coronal initiation, (2) the longitudinal span of the CME to be readily charted, and (3) the transporting speed of the CME to be reliably determined. Additional WL polarization measurements can significantly limit the ambiguity of localizing CMEs. We assert that a panoramic OOE view in WL would be highly beneficial in revealing CME morphology and kinematics in the hitherto-unresolved longitudinal dimension and hence for monitoring the propagation and evolution of near-ecliptic CMEs for space weather operations.

Impact of non-migrating tides on the low latitude ionosphere during a sudden stratospheric warming event in January 2010

The lower atmosphere contributes significantly to the day-to-day variability of the ionosphere, especially during solar minimum conditions. Ionosphere/atmosphere model simulations that incorporate meteorology from data assimilation analysis products can be critically important for elucidating the physical processes that have substantial impact on ionospheric weather. In this study, the NCAR Whole Atmosphere Community Climate Model, extended version with specified dynamics (SD-WACCM-X) is coupled with an ionospheric model (Sami3 is Another Model of the Ionosphere) to study day-to-day variability in the ionosphere during January 2010. Lower atmospheric weather patterns are introduced into the SAMI3/SD-WACCM-X simulations using the 6-h Navy Operational Global Atmospheric Prediction System-Advanced Level Physics High Altitude (NOGAPS-ALPHA) data assimilation products. The same time period is simulated using the new atmospheric forecast model, the High Altitude Navy Global Environmental Model (HA-NAVGEM), a hybrid 4D-Var prototype data assimilation with the ability to produce meteorological fields at a 3-h cadence. Our study shows that forcing SD-WACCM-X with HA-NAVGEM better resolves the semidiurnal tides and introduces more day-to-day variability into the ionosphere than forcing with NOGAPS-ALPHA. The SAMI3/SD-WACCM-X/HA-NAVGEM simulation also more accurately captures the longitudinal variability associated with non-migrating tides in the equatorial ionization anomaly (EIA) region as compared to total electron content (TEC) maps derived from GPS data. Both the TEC maps and the SAMI3/SD-WACCM-X/HA-NAVGEM simulation show an enhancement in TEC over South America during 17–21 January 2010, which coincides with the commencement of a stratospheric warming event on 19 January 2010. Analysis of the SAMI3/SD-WACCM-X/HA-NAVGEM simulations indicates non-migrating tides (including DW4, DE2 and SW5) played a role during 17–21 January in shifting the phase of the wave-3 pattern in the ionosphere on these days. Constructive interference of wave-3 and wave-4 patterns in the E × B drifts contributed to the enhanced TEC in the South American longitude sector. The results of the study highlight the importance of high fidelity meteorology in understanding the day-to-day variability of the ionosphere.

Nonlinear whistler wave model for lion roars in the Earth’s magnetosheath

In the present paper, we investigate the power-law behaviour of the magnetic field spectra in the Earths magnetosheath region using Cluster spacecraft data under solar minimum condition. The power spectral density of the magnetic field data and spectral slopes at various frequencies are analysed. Propagation angle and compressibility are used to test the nature of turbulent fluctuations. The magnetic field spectra have the spectral slopes between -1.5 to 0 down to spatial scales of 20 ion gyroradius and show clear evidence of a transition to steeper spectra for small scales with a second power-law, having slopes between -2.6 to -1.8. At low frequencies, f_sc<0.3f_ci(where f_ci is ion gyro-frequency), propagation angle approximately 90 degrees to the mean magnetic field, B_0, and compressibility shows a broad distribution, 0.1 < R > 0.9. On the other hand at f_sc>10f_ci, the propagation angle exhibits a broad range between 30-90 degree while 'R' has a small variation: 0.2 < R > 0.5. We conjecture that at high frequencies, the perpendicularly propagating Alfven waves could partly explain the statistical analysis of spectra. To support our prediction of kinetic Alfven wave-dominated spectral slope behaviour at high frequency, we also present a theoretical model and simulate the magnetic field turbulence spectra due to the nonlinear evolution of kinetic Alfven waves. The present study also shows the analogy between the observational and simulated spectra.

The Slowly Varying Corona. I. Daily Differential Emission Measure Distributions Derived from EVE Spectra

Abstract Daily differential emission measure (DEM) distributions of the solar corona are derived from spectra obtained by the Extreme-ultraviolet Variability Experiment (EVE) over a 4 yr period starting in 2010 near solar minimum and continuing through the maximum of solar cycle 24. The DEMs are calculated using six strong emission features dominated by Fe lines of charge states viii, ix, xi, xii, xiv, and xvi that sample the nonflaring coronal temperature range 0.3–5 MK. A proxy for the non-Fe xviii emission in the wavelength band around the 93.9 Å line is demonstrated. There is little variability in the cool component of the corona (T &lt; 1.3 MK) over the 4 yr, suggesting that the quiet-Sun corona does not respond strongly to the solar cycle, whereas the hotter component (T &gt; 2.0 MK) varies by more than an order of magnitude. A discontinuity in the behavior of coronal diagnostics in 2011 February–March, around the time of the first X-class flare of cycle 24, suggests fundamentally different behavior in the corona under solar minimum and maximum conditions. This global state transition occurs over a period of several months. The DEMs are used to estimate the thermal energy of the visible solar corona (of order 1031 erg), its radiative energy loss rate ((2.5–8) erg s−1), and the corresponding energy turnover timescale (about an hour). The uncertainties associated with the DEMs and these derived values are mostly due to the coronal Fe abundance and density and the CHIANTI atomic line database.

Electron density extrapolation above F2 peak by the linear Vary‐Chap model supporting new Global Navigation Satellite Systems‐LEO occultation missions

AbstractThe new radio‐occultation (RO) instrument on board the future EUMETSAT Polar System‐Second Generation (EPS‐SG) satellites, flying at a height of 820 km, is primarily focusing on neutral atmospheric profiling. It will also provide an opportunity for RO ionospheric sounding, but only below impact heights of 500 km, in order to guarantee a full data gathering of the neutral part. This will leave a gap of 320 km, which impedes the application of the direct inversion techniques to retrieve the electron density profile. To overcome this challenge, we have looked for new ways (accurate and simple) of extrapolating the electron density (also applicable to other low‐Earth orbiting, LEO, missions like CHAMP): a new Vary‐Chap Extrapolation Technique (VCET). VCET is based on the scale height behavior, linearly dependent on the altitude above hmF2. This allows extrapolating the electron density profile for impact heights above its peak height (this is the case for EPS‐SG), up to the satellite orbital height. VCET has been assessed with more than 3700 complete electron density profiles obtained in four representative scenarios of the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) in the United States and the Formosa Satellite Mission 3 (FORMOSAT‐3) in Taiwan, in solar maximum and minimum conditions, and geomagnetically disturbed conditions, by applying an updated Improved Abel Transform Inversion technique to dual‐frequency GPS measurements. It is shown that VCET performs much better than other classical Chapman models, with 60% of occultations showing relative extrapolation errors below 20%, in contrast with conventional Chapman model extrapolation approaches with 10% or less of the profiles with relative error below 20%.

Deciphering Solar Magnetic Activity: Spotting Solar Cycle 25

We present observational signatures of solar cycle 25 onset. Those signatures are visibly following a migratory path from high to low latitudes. They had starting points that are asymmetrically offset in each hemisphere at times that are 21-22 years after the corresponding, same polarity, activity bands of solar cycle 23 started their migration. Those bands define the so-called extended solar cycle. The four magnetic bands currently present in the system are approaching a mutually cancelling configuration, and solar minimum conditions are imminent. Further, using a tuned analysis of the daily band latitude-time diagnostics, we are able to utilize the longitudinal wave number (m=1) variation in the data to more clearly reveal the presence of the solar cycle 25 bands. This clarification illustrates that prevalently active longitudes (different in each hemisphere) exist at mid-latitudes presently, lasting many solar rotations, that can be used for detailed study over the next several years with instruments like the Spectrograph on IRIS, the Spectropolarimeter on Hinode, and, when they come online, similar instruments on the Daniel K. Inouye Solar Telescope (DKIST) as we watch those bands evolve following the cancellation of the solar cycle 24 activity bands at the equator late in 2019.

Nonmigrating tidal impact on the CO2 15 μm infrared cooling of the lower thermosphere during solar minimum conditions

AbstractCarbon dioxide (CO2) infrared emissions at 15 μm is the primary radiative cooling mechanism of the thermosphere in the altitude range of 100–135 km. This paper explores the role of two important diurnal nonmigrating tides, the DE2 and DE3, in the modulation of CO2 15 μm emissions during the solar minimum year 2008 by (i) analyzing Sounding the Atmosphere using Broadband Emission Radiometry (SABER) CO2 cooling rate data and (ii) photochemical modeling using dynamical tides from the empirical Climatological Tidal Model of the Thermosphere model. Tidal diagnostics of SABER data shows that the CO2 cooling rate amplitudes for the DE2 and DE3 components are on the order of approximately 20–50% relative to the monthly means, and they maximize around the lower bound (100 km) of the analyzed height interval. The photochemical modeling reproduces the observed results, albeit with systematic amplitude differences which is likely related to the uncertainty in the model input backgrounds, especially atomic oxygen. The main tidal coupling mechanism is found to be the temperature dependence of the collisional excitation of the CO2 ν2 vibrational state. However, neutral density becomes equally important above ∼110 km, thereby explaining observed evanescent DE2 and DE3 phases which are not present in temperature tides. The contribution of vertical tidal advection is comparatively small. The relative importance of the coupling mechanisms is the same at all latitudes/seasons. These results indicate that upward propagating nonmigrating tides forced by latent heat release in the lower atmosphere impact the thermospheric energy budget by modulating the longitudinal/local time behavior of the CO2 infrared cooling.