High Magnetic Latitudes Research Articles

Quantifying Extreme Values in Geomagnetic Perturbations Using Ground Magnetic Records

AbstractWe comprehensively analyzed geomagnetic perturbations using ground magnetic records from over 400 stations spanning four solar cycles, from 1976 to 2023. We assess the perturbations in the three magnetic components separately. Our study covers low, middle, and high magnetic latitudes in the northern magnetic hemisphere, with the primary objective of quantifying extreme values and evaluating their variability on magnetic latitude, local time, and solar cycle phases “minimum, ascending, maximum, and declining.” Our findings reveal spatial patterns to be less discernible as perturbations intensify, with distinct responses at middle and high latitudes. The extreme values, defined as percentiles 0 and 100, were observed to be localized and randomly distributed in local time, especially in the east magnetic component. Additionally, we observed dusk‐dawn asymmetries in the magnitude of perturbations related to the auroral electrojets, indicating complex interactions between the magnetosphere and ionosphere. Furthermore, the results reveal a preference for the most significant extreme values to occur in the declining phase of the solar cycle. These insights deepen our understanding of geomagnetic perturbations and their variability, contributing to space weather forecasting and mitigation strategies.

Prediction of high latitude nightside Geomagnetic Field Disturbances (GGFDs) during the 9-March-2012 geomagnetic storm using Recurrent Neural Network (RNN)

Geomagnetically Induced currents (GICs) pose a potentially catastrophic threat to the safety and security of large-scale power distribution systems. Using a recurrent neural network, we predict high (≥60°) magnetic latitude (MLAT) and night side (06–18) magnetic local time (MLT) geomagnetic field disturbances (surface dBs/dt and vertical dBz/dt), the trigger of GICs, during a geomagnetic storms occurred on March 09, 2012. We use the high latitude geomagnetic field data from SuperMAG and the solar wind particle and magnetic field data from NASA-OMNIWeb. We divide the high latitude night side of Earth’s globe into multiple sectors and obtain a time series of maximum geomagnetic disturbances observed in each sector. We use the Long Short Term Memory (LSTM) network technique sector by sector on both, the solar wind data and the sectored geomagnetic field disturbance (10° geomagnetic latitude, 1-hour MLT) time series obtained from SuperMAG. Each sector prediction is plotted on a polar map to reveal high latitude night side geomagnetic disturbance patterns. This high latitude nigh side model has great potential. With solar wind input at a Lagrangian-1 point, and geomagnetic field data from SuperMAG, it can predict the peak magnetic disturbance at any given sector for couple of hours in advance. Our machine learning techniques provide the best set of solar wind input that gives the best disturbance prediction. This paper presents a prototype of the global geomagnetic disturbance model with low-resolution spatial grids and a subset of potential solar wind input parameters. Once matured, the global magnetic disturbance model will be provided to the public.

Observation of an Electron Microburst With an Inverse Time‐Of‐Flight Energy Dispersion

AbstractInteractions between whistler mode chorus waves and electrons are a dominant mechanism for particle acceleration and loss in the outer radiation belt. One form of this loss is electron microburst precipitation: a sub‐second intense burst of electrons. Despite previous investigations, details regarding the microburst‐chorus scattering mechanism—such as dominant resonance harmonic—are largely unconstrained. One way to observationally probe this is via the time‐of‐flight energy dispersion. If a single cyclotron resonance is dominant, then higher energy electrons will resonate at higher magnetic latitudes: sometimes resulting in an inverse time‐of‐flight dispersion with lower‐energy electrons leading. Here we present a clear example of this phenomena, observed by a FIREBIRD‐II CubeSat on 27 August 2015, that shows good agreement with the Miyoshi‐Saito time‐of‐flight model. When constrained by this observation, the Miyoshi‐Saito model predicts that a relatively narrowband chorus wave with a ∼0.2 of the equatorial electron gyrofrequency scattered the microburst.

Magnetic Tilt Effect on Externally Driven Electromagnetic Ion Cyclotron (EMIC) Waves

AbstractWe examine coupling of fluctuations in the solar wind with electromagnetic ion cyclotron (EMIC) waves in the magnetosphere using an advanced full‐wave simulation code, Petra‐M. Dipole tilt dramatically affects the coupling process. While very little wave power can reach the inner magnetosphere without tilt effects, a tilted dipole field dramatically increases the efficiency of the coupling process. Solar wind fluctuations incident at high magnetic latitude effectively reaches the ground along the field line and mode‐convert to linearly polarized field‐aligned propagating waves at the Alfvén and IIH resonances. Therefore, solar wind compressions efficiently drive linearly polarized EMIC waves when the dipole angle is tilted toward or away from the Sun‐Earth direction.

Midlatitude Sporadic E‐Layer Horizontal Structuring Modulated by Neutral Instability and Mixing in the Lower Thermosphere

AbstractObservations of 30‐MHz coherent backscatter from sporadic‐E ionization layers were obtained with a VHF imaging radar located in Ithaca, New York. The volume probed by the radar lies at relatively high magnetic latitudes, on the northern edge of the mid‐latitude region and underneath the ionospheric trough. Banded, quasi‐periodic (QP) echoes observed from Ithaca are similar to those found in lower midlatitude regions. The Doppler shifts observed are smaller and, so far, do not appear to reach the threshold for Farley‐Buneman instability. However, many of the echoes exhibit fine‐scale structure, with secondary bands or braids oriented obliquely to the primary bands. Secondary bands have been seen only rarely at lower middle latitudes. In previous observations, the QP scattering has been linked to unstable neutral wind shears. Neutral wind shear commonly found in the lower thermosphere could play a key role in the formation of these irregularities and explain some morphological features of the resulting plasma density irregularities and the radar echoes. We consider whether neutral instability and turbulence in the lower thermosphere is the likely cause for some of the structuring in the sporadic‐ E layers. Results of 3D numerical simulations of atmospheric dynamics in the mesosphere to lower thermosphere support the proposition. In particular, we focus on Ekman‐type instabilities that, like the more common Kelvin‐Helmholtz instabilities, are inflection point instabilities, although specifically associated with turning shears, and result in convective rolls aligned close to the mean wind direction, with smaller‐scale secondary waves aligned normal to the primary structures.

Power-law dependence of the wavelet spectrum of ground magnetic variations during magnetic storms

We show that time variations in the ring current and auroral electrojets in 2015, during which three major magnetic storms (MSs) occurred, are recorded in several specific ways by ground magnetometers. Specifically, we show that the time variations in magnetic field intensity have power-law wavelet spectra. The variations in ring current intensity are detected in geomagnetic field measurements of ground stations at magnetic mid-latitudes between λ=-50° and +50°. We confirmed the results of Balasis et al. (2006), who had shown that the Dst index wavelet spectrum follows a power law of the form f-β, where f is the frequency, and β is the exponent of the power law, by applying the same wavelet analysis to the SYM-H index version of the Dst index, and also to ground magnetic field measurements. Balasis et al. (2006) also showed that the critical exponent β of the power law, obtains values greater than 2 (or equivalently that the Hurst exponent H>0.5), shortly before the onset and during the main phase of the storms. Here, using wavelet transforms on the Earth’s magnetic field as measured at different magnetic latitudes (MLAT), we showed that at mid-geomagnetic latitudes, where the Earth’s magnetic field is determined mainly by the ring current, the resulting Hurst exponent is greater than 0.5 several hours before and during magnetic storms. Geomagnetic field variation at higher magnetic latitudes between λ=+50° and +73°, also show power-law wavelet spectra. However, here we find a more complex dependence with two power-law exponents. We interpret this double power law as the effect of both the ring current and the auroral electrojets. Finally, for polar latitudes, between λ=+73° and +85°, we find the same complex dependence with two power-law exponents. Our results show that the geomagnetic field at auroral latitudes has much lower power and very different wavelet spectra compared to the geomagnetic field at ranges of lower magnetic latitude. Our results show how wavelet-based measures of ground geomagnetic variations reflect the time evolution of several magnetospheric current systems.

Day-to-Day Variability of the Semidiurnal Tide in the F-Region Ionosphere During the January 2021 SSW From COSMIC-2 and ICON.

The semidiurnal tidal spectrum in the F‐region ionosphere obtained from hourly COSMIC‐2 Global Ionospheric Specification (GIS) data assimilation is greatly (>50%) enhanced during the January 2021 Sudden Stratospheric Warming (SSW). Moreover, the semidiurnal migrating tidal response in topside electron densities closely follows the day‐to‐day changes of the 10 hPa, 60°N zonal wind from MERRA‐2 during the SSW. The response is similar in the northern and southern crests of the Equatorial Ionization Anomaly (EIA) but persists toward higher magnetic latitudes and the EIA trough. A slight phase shift toward earlier local times is consistent with theoretical expectations of an E‐region dynamo driving and agrees with semidiurnal tidal diagnostics of MIGHTI/ICON zonal winds at 105 km. COSMIC‐2 GIS are the first data set to resolve the tidal weather of the ionosphere on a day‐to‐day basis and, therefore, provide a new perspective on space weather variability driven by lower and middle atmosphere dynamics.

Inter‐Calibration Between the Electron Flux Measurements of FengYun‐3B and Van Allen Probe‐A Based on Electron Phase Space Density Conjunctions

AbstractCross‐satellite calibration of energetic particle fluxes is essential to understanding Earth's radiation belt dynamics and modeling the space radiation environment. Using the method of comparing phase space density (PSD) conjunctions in the same set of phase space coordinates, we perform a cross‐satellite calibration of energetic electron fluxes measured by the Low Earth Orbit Chinese FengYun‐3B satellite (FY‐3B) and the near‐equatorial Highly Eccentric Orbit Van Allen Probe‐A (VAP‐A), respectively. VAP‐A provides pitch angle resolved electron (directional) differential fluxes while FY‐3B provides omnidirectional electron differential fluxes. Before calculating the PSDs from FY‐3B high energy electron detector (HEED), a method is introduced to estimate the local pitch angle of FY‐3B based on the installation direction information of HEED and the T89c magnetic field model. We calculate the calibration factors by using the PSD conjunctions inferred from the two satellites for three fixed sets of (μ, K, L*) during the geomagnetic storm times. The calibration factors range from 0.76 to 0.86 varying with fixed sets of (μ, K, L*), which indicates that the electron differential fluxes from FY‐3B are higher than that from VAP‐A. During geomagnetic quiet times, we perform calibration by using independent Gaussian fitting analysis of the number of points with the unaveraged PSDs of FY‐3B and VAP‐A for three fixed sets of (μ, K, L*), with the corresponding calibration factors of 0.78, 0.72, and 0.46. The calibrated electron differential fluxes of FY‐3B measured at the high magnetic latitudes, can be used in the future modeling of the radiation belt electron dynamics.

Research on the On-orbit Background of the Hard X-Ray Imager Onboard ASO-S

The space environment background of various particle fluxes of the Hard X-ray Imager (HXI), one of the payloads of the Advanced Space-based Solar Observatory (ASO-S) spacecraft, is investigated and presented. Different approaches are used to obtain the input information on various space environment particles (protons, alpha particles, electrons, positrons, neutrons, and photons). Some special regions (SAA and radiation belt) are also taken into account. The findings indicate that electrons are the primary background source in the radiation belt. Due to the large background flux generated by electrons, HXI cannot effectively observe solar flares in the radiation belt. Outside the radiation belt, primary protons and albedo photons are the main sources of background at low and high magnetic latitudes respectively. The statistical analysis of the flare and background spectra shows that the errors of the flare energy spectrum observation are mainly concentrated in the high energy band, and the detector still has a certain spectrum observation capability for flares of C-class and below in the low energy band of the non-radiation belt. The imaging observation of flares of C-class and below is significantly affected by the accuracy of background subtraction. The energy band with the best signal-to-noise ratio is from 10 to 50 keV, which can be used to monitor the formation and class of flares.

Quasi‐Periodic Whistler Mode Emission in the Plasmasphere as Observed by the DSX Spacecraft

AbstractWe describe the quasi‐periodic (QP) whistler‐mode emissions found in the plasmasphere as detected by electric and magnetic instrumentation onboard the Demonstration and Science Experiments (DSX) spacecraft in medium Earth orbit. Over the course of the nearly 2‐year mission, at least 45 episodes of whistler mode QP emissions were detected by the Broad Band Receiver (BBR) onboard DSX. Episodes of QP emissions were identified by discrete events having a clear unambiguous periodic nature as detected by both the electric antennae and search coil magnetic sensors in the BBR survey data at 30 s temporal resolution. Most of the QP episodes occurred in a frequency range between 1 and 4 kHz, in a band previously identified by Van Allen Probes and Cluster investigators. However, episodes were also detected by DSX at higher frequencies ‐ events in these episodes extending all the way to 15 kHz. We present our findings on these unusual high frequency events in the presentation herein. Specifically, these high frequency QP episodes tended to be observed near dawn/dusk when the spacecraft was at relatively high magnetic latitudes and on magnetic L‐shells between 3 and 5. Another unusual feature of these episodes is that individual up‐drifting events making up the episode were found to sometimes occur concurrently in time: The high frequency portion of one up‐drifting “polliwog‐shaped” event overlapped in time with the low frequency portion of the subsequent event. This behavior of the QP emissions has not been previously emphasized and we consider how this temporal concurrence relates to the source processes.

Magnetospheric Ion Bombardment of Europa’s Surface

Abstract Jupiter’s moon Europa is exposed to constant bombardment by magnetospheric charged particles, which are expected to be a major source of physical and chemical surface modification. Here we have investigated the flux of magnetospheric ions at Europa’s surface by carrying out single particle tracing within realistic electromagnetic fields from multifluid magnetohydrodynamic simulations of the moon’s interaction with Jupiter’s magnetosphere. We find that magnetic field line draping and pileup leads to shielding and drastically reduced flux at low latitudes across Europa’s trailing (upstream) hemisphere. Furthermore, we find that magnetic induction within Europa’s subsurface ocean leads to additional shielding when the moon is located at high magnetic latitudes in Jupiter’s magnetosphere. Overall, we find that the high-latitude and polar regions on Europa receive the largest flux of magnetospheric ions. Both spacecraft and ground-based observations have previously identified a non–water ice surface species concentrated at Europa’s trailing (upstream) hemisphere, possibly hydrated sulfuric acid formed from radiolysis of water ice with implanted S ions. Our results demonstrate that the S ion flux across Europa’s equatorial trailing (upstream) hemisphere is strongly reduced, possibly indicating that the formation of the observed non–water ice species is controlled primarily by energy input from magnetospheric electrons, rather than the flux of S ions. We find that that O and S ions at >1 MeV energies have nearly uniform access to the surface, while energetic protons in this energy range are constrained to a “bull’s-eye” centered on the trailing (upstream) hemisphere.

Empirical Selection of Auroral Kilometric Radiation During a Multipoint Remote Observation With Wind and Cassini

AbstractAuroral Kilometric Radiation (AKR) is terrestrial radio emission that originates in particle acceleration regions along magnetic field lines, coinciding with discrete auroral arcs. AKR viewing geometry is complex due to the confinement of the source regions to nightside local times (LTs) and the anisotropy of the beaming pattern, so observations are highly dependent on spacecraft viewing position. We present a novel, empirical technique that selects AKR emission from observations made with the spin‐axis aligned antenna of the Wind/WAVES instrument, based on the rapidly varying amplitude of AKR across spacecraft spin timescales. We apply the technique to Wind/WAVES data during 1999 day of year 227–257, when the Cassini spacecraft flew past Earth and provided an opportunity to observe AKR from two remote locations. We examine the AKR flux and power, with observations made from LTs of 1700–0300 hr having an average power up to 104 Wsr‐1 larger than those on the dayside and an increasing AKR power observed at higher magnetic latitudes. We perform a linear cross‐correlation between the Wind AKR power and the spacecraft magnetic latitude, showing positive then negative correlation as Wind travels from the Northern to Southern magnetic hemisphere. Statistically significant diurnal modulations are found in the whole 30‐day period and in subsets of the data covering different local time sectors, indicative of a predominantly geometrical effect for remote AKR viewing. The reproduction of well‐known features of the AKR verifies the empirical selection and shows the promise of its application to Wind/WAVES observations.

Seasonal and Solar Wind Sector Duration Influences on the Correlation of High Latitude Clouds With Ionospheric Potential

AbstractIrradiances from long‐lived stratus‐type clouds at Alert (Canada), Summit (Greenland), and South Pole, previously measured, show correlations with the day‐to‐day input to the global atmospheric electric circuit from the solar wind, as well as with the inputs of low‐ and mid‐latitude thunderstorms and shower clouds. We analyze the measured Alert cloud irradiances, and find differences in the responses to 2, 4, or more solar wind sectors per 27‐days solar rotation. We find seasonal variations in the correlations, with sign reversal in the summer. The correlation coefficients that were found previously for all‐year, all sector types show further increases for just winter months and in addition, for just 2‐sector intervals. At high magnetic latitudes, the ionospheric potential correlates strongly with the solar wind sector structure, and determines the flow of current density (JZ) to the Earth's surface that passes through clouds and modifies space charge in them. Parameterizations of the potential distribution near the magnetic pole are used in the correlations. The daily average values depend mainly on the solar wind (interplanetary) magnetic field (IMF) BY component, with lesser influence of the solar wind speed and IMF BZ. Mechanisms by which space charge in clouds can affect cloud microphysics and cloud opacity are described and are qualitatively consistent with the correlations, but need quantitative testing.

E‐POP Observations of Suprathermal Electron Bursts in the Ionospheric Alfvén Resonator

AbstractUtilizing the suprathermal electron imager onboard the e‐POP satellite, we performed a statistical study of groups of repetitive suprathermal electron bursts (STEBs) with 52 events (each with ≥3 STEBs) during 21 e‐POP orbits. The STEBs are observed on both the day side and night side, and within a wide range of geomagnetic latitudes, from 64° to 78°. We interpret these data as temporally varying signatures; we suggest the repetition is due to the ionospheric Alfvén resonator (IAR), since the statistical time separation of adjacent STEBs in the satellite frame is 0.5–1.5 s, which is consistent with the characteristic period of the IAR. Furthermore, the STEBs are associated with low‐frequency magnetic field perturbations. The statistical occurrence of repetitive STEBs shows a preference toward postnoon and midnight sectors at high magnetic latitudes. We also present one example event of four successive STEBs associated with auroral rays in conjugate imagery. The repetition period of STEBs is similar to the oscillation period of magnetic fields observed by Enhanced Polar Outflow Probe (e‐POP). One of the STEBs in an example event exhibits “normal” dispersion (high energy particles with field‐aligned pitch angle arriving first) and the adjacent STEB exhibits “inverse” dispersion (low energy particles with broad pitch angles arriving first). With a test particle simulation, we propose one possible explanation of inverse dispersion in terms of a suprathermal electron population and a downward‐propagating parallel electric field.

Pro‐L* ‐ A Probabilistic L* Mapping Tool for Ground Observations

AbstractBoth ground and space observations are used extensively in the modeling of space weather processes within the Earth’s magnetosphere. In radiation belt physics modeling, one of the key phase‐space coordinates is L*, which indicates the location of the drift paths of energetic electrons. Global magnetic field models allow a subset of locations on the ground (mainly subauroral) to be mapped along field lines to a location in space and transformed into L*, provided that the initial ground location maps to a closed drift path. This allows observations from ground, or low‐altitude space‐based platforms to be mapped into space in order to inform radiation belt modeling. Many data‐based magnetic field models exist; however, these models can significantly disagree on mapped L* values for a single point on the ground, during both quiet times and storms. We present a state of the art probabilistic L* mapping tool, Pro‐L*, which produces probability distributions for L* corresponding to a given ground location. Pro‐L* has been calculated for a high resolution magnetic latitude by magnetic local time grid in the Earth’s Northern Hemisphere. We have developed the probabilistic model using 11 years of L* calculations for seven widely used magnetic field models. Usage of the tool is highlighted for both event studies and statistical models, and we demonstrate a number of potential applications.

Analysis and Attribution of Climate Change in the Upper Atmosphere From 1950 to 2015 Simulated by WACCM‐X

AbstractMonitoring climatic changes in the thermosphere and ionosphere and understanding their causes is important for practical purposes. To support this effort and facilitate comparisons between observations and model results, a long transient simulation with the Whole Atmosphere Community Climate Model eXtension (WACCM‐X) from 1950 to 2015 was conducted. This simulation used realistic variations in solar and geomagnetic activity, main magnetic field changes, and trace gas emissions, including CO2, thereby including all known drivers of upper atmosphere climate change. Analysis of the full 1950–2015 interval with a standard multilinear regression approach demonstrated difficulties in removing solar cycle effects sufficiently to obtain reliable trends. Results improved when an (F10.7a)2 was included in the regression model, in addition to terms for F10.7a, KP, and the trend itself. Comparisons with previous studies and analysis of spatial variations in trend estimates confirmed that the increase in CO2 concentration is the main driver of trends in thermosphere temperature and density, but at high (magnetic) latitudes effects of main magnetic field changes play a role as well, especially in the Northern Hemisphere. Spatial patterns of trends in hmF2, NmF2, and total electron content indicate a superposition of CO2 and geomagnetic field effects, with the latter dominating trends in the region of ∼50–20°N, ∼60°W to 20°E. Additional model experiments to investigate the indirect dynamical effects of climate change in the lower atmosphere (<50 km) on the upper atmosphere (>100 km) suggested that these effects are small and insignificant. However, current model limitations could mean that these effects are underestimated.

Echo of ring current storms in the ionosphere

The ionospheric weather Vσ index maps are generated from GIM-TEC produced at the Jet Propulsion Laboratory (JPL) with the denser grids of 1 × 1° in latitude and longitude, respectively. Using sliding-window statistical analysis, moving TEC median μ for 15 preceding days with estimated variance bounds is obtained for each grid pixel of GIM-TEC. The ionosphere variability index, Vσ, is ΔTEC deviation from the median normalized by the standard deviation σ. Vσ index segmentation is introduced specifying TEC storm if an instant TEC is outside the bound of μ±2σ. Comparisons of Vσ variability of 1 × 1° GIM-TEC maps with normal GIM-TECs of 2.5 × 5° in latitude and longitude demonstrates the greater TEC storm activity with denser grids maps. The superposed epoch analysis is performed for 77 intense storms (Dst ≤ −100 nT) and 246 moderate storms (−100 < Dst ≤ −50 nT) during 2000–2017, with zero time, t0, at Dst storm peak. The probabilities of TEC storm occurrence both positive (Vσp index) and negative (Vσn) are evaluated for 24 h prior to t0 and 48 h after t0 for global scale and three specified latitude zones: North High (NH) magnetic latitudes λ > 50°, South High (SH) λ < −50°, and Equatorial Ionization Anomaly (EIA) Band between ±35° magnetic latitudes. The equinoctial positive Vσp index shows the maxima on March and September, Dst intensity peaks on April and October, and the negative Vσn peaks on May and November. The amplitude of Vσp for the intense storms exceeds that for the moderate storm by 2 times and exceeds the amplitude of Vσn by 1.5 times. Storm Enhanced Density (SED) is evident with the largest amplitude of Vσp at EIA Band exceeding Vσp for NH and SH zones by 3 times. The negative Vσn occurrence observed for selected zones during the intense storms is decreased by 5 times for the moderate storms. The normalized cross-correlation function is used to define lag (time delay) between peak of Vσp and Vσn and t0 of Dst peak. The majority of positive Vσp indices (57.5%) are observed prior to Dst storm (lag varies from −1 to −6 h with Dst from −50 to −400 nT) so the TEC enhancement can serve as a precursor of Dst storm. The time delay for the negative Vσn is inversely related with Dst storm intensity (lag varies from 11 to 8 h with Dst from −50 to −400 nT).

Global‐Scale Observations and Modeling of Far‐Ultraviolet Airglow During Twilight

AbstractThe NASA Global‐scale Observations of the Limb and Disk ultraviolet imaging spectrograph performs observations of upper atmosphere airglow from the sunlit disk and limb of the Earth, in order to infer quantities such as the composition and temperature of the thermosphere. To interpret the measurements, the observational and solar illumination geometry must be considered. We use forward models of upper atmosphere density and composition, photoelectron impact, airglow emissions, radiative transfer, and line‐of‐sight integration, to describe the expected observations, and here test those calculations against observations near the terminator, and near the limb. On the nightside of the terminator, broad regions of faint airglow are seen, particularly near the winter solstice. These are caused by photoelectrons generated in magnetically conjugate areas in the other hemisphere that are still illuminated, transported along field lines, and then precipitated back into the atmosphere. Model calculations demonstrate that this process is the source of the emission, and obtain good agreement with its morphology and intensity. In some regions, the observed emissions are not as bright as the model simulations. Some of the reduction in electron flux is explained by changes in magnetic field strength; in other cases, particularly at high magnetic latitude, the cause is unknown, but must occur along extended field lines that reach into the plasma sheet.

MESSENGER Observations of Disappearing Dayside Magnetosphere Events at Mercury

AbstractMErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) measurements taken during passes over Mercury's dayside hemisphere indicate that on four occasions the spacecraft remained in the magnetosheath even though it reached altitudes below 300 km. During these disappearing dayside magnetosphere (DDM) events, the spacecraft did not encounter the magnetopause until it was at very high magnetic latitudes, ~66 to 80°. These DDM events stand out with respect to their extremely high solar wind dynamic pressures, Psw ~140 to 290 nPa, and intense southward magnetic fields, Bz ~ −100 to −400 nT, measured in the magnetosheath. In addition, the bow shock was observed very close to the surface during these events with a subsolar altitude of ~1,200 km. It is suggested that DDM events, which are closely associated with coronal mass ejections, are due to solar wind compression and/or reconnection‐driven erosion of the dayside magnetosphere. The very low altitude of the bow shock during these events strongly suggests that the solar wind impacts much of Mercury's sunlit hemisphere during these events. More study of these disappearing dayside events is required, but it is likely that solar wind sputtering of neutrals from the surface into the exosphere maximizes during these intervals.

Statistical Analysis of Transverse Size of Lower Band Chorus Waves Using Simultaneous Multisatellite Observations

AbstractChorus waves are known to accelerate or scatter energetic electrons via quasi‐linear or nonlinear wave‐particle interactions in the Earth's magnetosphere. In this letter, by taking advantage of simultaneous observations of chorus waveforms from at least a pair of probes among Van Allen Probes and/or Time History of Events and Macroscale Interactions during Substorms (THEMIS) missions, we statistically calculate the transverse size of lower band chorus wave elements. The average size of lower band chorus wave element is found to be ~315±32 km over L shells of ~5–6. Furthermore, our results suggest that the scale size of lower band chorus tends to be (1) larger at higher L shells; (2) larger at higher magnetic latitudes, especially on the dayside; and (3) larger in the azimuthal direction than in the radial direction. Our findings are crucial to quantify wave‐particle interaction process, particularly the nonlinear interactions between chorus and energetic electrons.