Function Of Geomagnetic Activity Research Articles

On the Interplay Between Solar Wind Parameters and ULF Wave Power as a Function of Geomagnetic Activity at High‐ and Mid‐latitudes

AbstractUsing ground magnetometer measurements from the IMAGE array over the timespan of one solar cycle, we apply wavelets to statistically assess the power of ultra‐low frequency (ULF) waves from L = 3.34 to L = 13.6 as a function of Kp, solar wind speed (), solar wind dynamic pressure (), Dst, and . We find that although geomagnetic storms account for the overwhelming majority of ULF wave power at mid latitudes, this effect decreases for higher L‐shells. We present analysis of the effect of interplanetary magnetic field (IMF) on ULF wave power for different values of Kp and , revealing a strong inter‐dependency especially in relation to . A parameterization additionally incorporating appears to be more important at large‐L. Further, analysis for positive reveals a perhaps unexpected additional dependence on which points to effects of the interplanetary magnetic field cone angle and Kelvin‐Helmholtz instabilities. Finally, we derive ULF wave radial diffusion coefficients from the measured ULF wave power spectral densities and briefly discuss their importance for radiation belt dynamics. Interestingly, in part because of the correlation between Dst‐defined storm times and high Kp, we find that the low‐Kp radial diffusion coefficients during storms are higher than previously assumed almost certainly because of the dominance of nonstorm epochs in the statistics of median ULF wave power under conditions of low Kp. These inter‐dependencies should be taken into account in future specification models for ULF wave radial diffusion coefficients when applied in storm time simulations.

Duration of Individual Relativistic Electron Microbursts: A Probe Into Their Scattering Mechanism

AbstractWe used the Solar Anomalous and Magnetospheric Particle Explorer to identify and quantify the duration of relativistic, MeV, electron microbursts. A typical relativistic microburst has a millisecond (ms) duration, and the interquartile range of the duration distribution is 70–140 ms. We investigated trends in the microburst duration as a function of geomagnetic activity, L‐shell, and magnetic local time (MLT). The clearest trend is in MLT: the median microburst duration doubles from 75 milliseconds at midnight to 140 milliseconds noon MLT. This trend is similar to the whistler mode chorus rising tone element duration trend, suggesting a possible relationship.

Analytical Chorus Wave Model Derived from Van Allen Probe Observations

AbstractChorus waves play an important role in the dynamic evolution of energetic electrons in the Earth's radiation belts and ring current. Using more than 5 years of Van Allen Probe data, we developed a new analytical model for upper‐band chorus (UBC; 0.5fce < f < fce) and lower‐band chorus (LBC; 0.05fce < f < 0.5fce) waves, where fce is the equatorial electron gyrofrequency. By applying polynomial fits to chorus wave root mean square amplitudes, we developed regression models for LBC and UBC as a function of geomagnetic activity (Kp), L, magnetic latitude (λ), and magnetic local time (MLT). Dependence on Kp is separated from the dependence on λ, L, and MLT as Kp‐scaling law to simplify the calculation of diffusion coefficients and inclusion into particle tracing codes. Frequency models for UBC and LBC are also developed, which depends on MLT and magnetic latitude. This empirical model is valid in all MLTs, magnetic latitude up to 20°, Kp ≤ 6, L‐shell range from 3.5 to 6 for LBC and from 4 to 6 for UBC. The dependence of root mean square amplitudes on L are different for different bands, which implies different energy sources for different wave bands. This analytical chorus wave model is convenient for inclusion in quasi‐linear diffusion calculations of electron scattering rates and particle simulations in the inner magnetosphere, especially for the newly developed four‐dimensional codes, which require significantly improved wave parameterizations.

NO<sub><i>y</i></sub> production, ozone loss and changes in net radiative heating due to energetic particle precipitation in 2002–2010

Abstract. We analyze the impact of energetic particle precipitation on the stratospheric nitrogen budget, ozone abundances and net radiative heating using results from three global chemistry-climate models considering solar protons and geomagnetic forcing due to auroral or radiation belt electrons. Two of the models cover the atmosphere up to the lower thermosphere, the source region of auroral NO production. Geomagnetic forcing in these models is included by prescribed ionization rates. One model reaches up to about 80 km, and geomagnetic forcing is included by applying an upper boundary condition of auroral NO mixing ratios parameterized as a function of geomagnetic activity. Despite the differences in the implementation of the particle effect, the resulting modeled NOy in the upper mesosphere agrees well between all three models, demonstrating that geomagnetic forcing is represented in a consistent way either by prescribing ionization rates or by prescribing NOy at the model top.Compared with observations of stratospheric and mesospheric NOy from the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) instrument for the years 2002–2010, the model simulations reproduce the spatial pattern and temporal evolution well. However, after strong sudden stratospheric warmings, particle-induced NOy is underestimated by both high-top models, and after the solar proton event in October 2003, NOy is overestimated by all three models. Model results indicate that the large solar proton event in October 2003 contributed about 1–2 Gmol (109 mol) NOy per hemisphere to the stratospheric NOy budget, while downwelling of auroral NOx from the upper mesosphere and lower thermosphere contributes up to 4 Gmol NOy. Accumulation over time leads to a constant particle-induced background of about 0.5–1 Gmol per hemisphere during solar minimum, and up to 2 Gmol per hemisphere during solar maximum. Related negative anomalies of ozone are predicted by the models in nearly every polar winter, ranging from 10–50 % during solar maximum to 2–10 % during solar minimum. Ozone loss continues throughout polar summer after strong solar proton events in the Southern Hemisphere and after large sudden stratospheric warmings in the Northern Hemisphere. During mid-winter, the ozone loss causes a reduction of the infrared radiative cooling, i.e., a positive change of the net radiative heating (effective warming), in agreement with analyses of geomagnetic forcing in stratospheric temperatures which show a warming in the late winter upper stratosphere. In late winter and spring, the sign of the net radiative heating change turns to negative (effective cooling). This spring-time cooling lasts well into summer and continues until the following autumn after large solar proton events in the Southern Hemisphere, and after sudden stratospheric warmings in the Northern Hemisphere.

Synthetic Empirical Chorus Wave Model From Combined Van Allen Probes and Cluster Statistics

AbstractChorus waves are among the most important natural electromagnetic emissions in the magnetosphere as regards to their potential effects on electron dynamics. They can efficiently accelerate or precipitate electrons trapped in the outer radiation belt, producing either fast increases of relativistic particle fluxes or auroras at high latitudes. Accurately modeling their effects, however, requires detailed models of their wave power and obliquity distribution as a function of geomagnetic activity in a particularly wide spatial domain, rarely available based solely on the statistics obtained from only one satellite mission. Here we seize the opportunity of synthesizing data from the Van Allen Probes and Cluster spacecraft to provide a new comprehensive chorus wave model in the outer radiation belt. The respective spatial coverages of these two missions are shown to be especially complementary and further allow a good cross calibration in the overlap domain. We used 4 years (2012–2016) of Van Allen Probes VLF data in the chorus frequency range up to 12 kHz at latitudes lower than 20°, combined with 10 years of Cluster VLF measurements up to 4 kHz in order to provide a full coverage of geomagnetic latitudes up to 45° in the chorus frequency range 0.1fce–0.8fce. The resulting synthetic statistical model of chorus wave amplitude, obliquity, and frequency is presented in the form of analytical functions of latitude and Kp in three different magnetic local time sectors and for two ranges of L shells outside the plasmasphere. Such a synthetic and reliable chorus model is crucially important for accurately modeling global acceleration and loss of electrons over the long run in the outer radiation belt, allowing a comprehensive description of electron flux variations over a very wide energy range.

First Study on the Occurrence Frequency of Equatorial Plasma Bubbles over West Africa Using an All‐Sky Airglow Imager and GNSS Receivers

AbstractThis is the first paper that reports the occurrence frequency of equatorial plasma bubbles and their dependences of local time, season, and geomagnetic activity based on airglow imaging observations at West Africa. The all‐sky imager, situated in Abuja (Geographic: 8.99°N, 7.38°E; Geomagnetic: 1.60°S), has a 180° fisheye view covering almost the entire airspace of Nigeria. Plasma bubbles are observed for 70 nights of the 147 clear‐sky nights from 9 June 2015 to 31 January 2017. Differences between nighttime and daytime ROTIs were also computed as a proxy of plasma bubbles using Global Navigation Satellite Systems (GNSS) receivers within the coverage of the all‐sky imager. Most plasma bubble occurrences are found during equinoxes and least occurrences during solstices. The occurrence rate of plasma bubbles was highest around local midnight and lower for hours farther away. Most of the postmidnight plasma bubbles were observed around the months of December to March, a period that coincides with the harmattan period in Nigeria. The on/off status of plasma bubble in airglow and GNSS observations were in agreement for 67.2% of the total 768 h, while we suggest several reasons responsible for the remaining 32.8% when the airglow and GNSS bubble status are inconsistent. A majority of the plasma bubbles were observed under relatively quiet geomagnetic conditions (Dst ≥ −40 and Kp ≤ 3), but there was no significant pattern observed in the occurrence rate of plasma bubbles as a function of geomagnetic activity. We suggest that geomagnetic activities could have either suppressed or promoted the occurrence of plasma bubbles.

Observation of chorus waves by the Van Allen Probes: Dependence on solar wind parameters and scale size

AbstractHighly energetic electrons in the Earth's Van Allen radiation belts can cause serious damage to spacecraft electronic systems and affect the atmospheric composition if they precipitate into the upper atmosphere. Whistler mode chorus waves have attracted significant attention in recent decades for their crucial role in the acceleration and loss of energetic electrons that ultimately change the dynamics of the radiation belts. The distribution of these waves in the inner magnetosphere is commonly presented as a function of geomagnetic activity. However, geomagnetic indices are nonspecific parameters that are compiled from imperfectly covered ground based measurements. The present study uses wave data from the two Van Allen Probes to present the distribution of lower band chorus waves not only as functions of single geomagnetic index and solar wind parameters but also as functions of combined parameters. Also the current study takes advantage of the unique equatorial orbit of the Van Allen Probes to estimate the average scale size of chorus wave packets, during close separations between the two spacecraft, as a function of radial distance, magnetic latitude, and geomagnetic activity, respectively. Results show that the average scale size of chorus wave packets is approximately 1300–2300 km. The results also show that the inclusion of combined parameters can provide better representation of the chorus wave distributions in the inner magnetosphere and therefore can further improve our knowledge of the acceleration and loss of radiation belt electrons.

Accurately characterizing the importance of wave-particle interactions in radiation belt dynamics: The pitfalls of statistical wave representations.

Wave‐particle interactions play a crucial role in energetic particle dynamics in the Earth's radiation belts. However, the relative importance of different wave modes in these dynamics is poorly understood. Typically, this is assessed during geomagnetic storms using statistically averaged empirical wave models as a function of geomagnetic activity in advanced radiation belt simulations. However, statistical averages poorly characterize extreme events such as geomagnetic storms in that storm‐time ultralow frequency wave power is typically larger than that derived over a solar cycle and Kp is a poor proxy for storm‐time wave power.

Electron scattering by magnetosonic waves in the inner magnetosphere

AbstractWe investigate the importance of electron scattering by magnetosonic waves in the Earth's inner magnetosphere. A statistical survey of the magnetosonic wave amplitude and wave frequency spectrum, as a function of geomagnetic activity, is performed using the Van Allen Probes wave measurements and is found to be generally consistent with the wave distribution obtained from previous spacecraft missions. Outside the plasmapause the statistical frequency distribution of magnetosonic waves follows the variation of the lower hybrid resonance frequency, but this trend is not observed inside the plasmasphere. Drift and bounce averaged electron diffusion rates due to magnetosonic waves are calculated using a recently developed analytical formula. The resulting timescale of electron energization during disturbed conditions (when AE* > 300 nT) is more than 10 days. We perform a 2‐D simulation of the electron phase space density evolution due to magnetosonic wave scattering during disturbed conditions. Outside the plasmapause, the waves accelerate electrons with pitch angles between 50° and 70° and form butterfly pitch angle distributions at energies from ~100 keV to a few MeV over a timescale of several days; whereas inside the plasmapause, the electron acceleration is very weak. Our study suggests that intense magnetosonic waves may cause the butterfly distribution of radiation belt electrons especially outside the plasmapause, but electron acceleration due to magnetosonic waves is generally not as effective as chorus wave acceleration.

Broadband low‐frequency electromagnetic waves in the inner magnetosphere

AbstractA prominent yet largely unrecognized feature of the inner magnetosphere associated with particle injections, and more generally geomagnetic storms, is the occurrence of broadband electromagnetic field fluctuations over spacecraft frame frequencies (fsc) extending from effectively zero to fsc ≳ 100 Hz. Using observations from the Van Allen Probes we show that these waves most commonly occur premidnight but are observed over a range of local times extending into the dayside magnetosphere. We find that the variation of magnetic spectral energy density with fsc obeys over several decades with a spectral breakpoint at fb ≈ 1 Hz. The values for α are lognormally distributed with α = 1.9 ± 0.6 for fsc < fb and α = 2.9 ± 0.6 for fsc > fb. A is a function of geomagnetic activity with the largest values observed over intervals of decreasing Dst index during the main phase of geomagnetic storms. At these times these waves are nearly always present in the nightside inner magnetosphere and are commonly observed from L = 3 outward. The observed variation of the electric to magnetic field amplitude with fsc is well described by a dispersive Alfvén wave model under the assumption that fsc is primarily a consequence of the Doppler shift of plasma frame structures moving over the spacecraft. The robust anticorrelation between the time rate change of the Dst index and wave spectral energy density coupled with the ability of dispersive Alfvén waves to drive transverse ion acceleration suggests that these waves may boost ion energy density in the inner magnetosphere and intensify the ring current during storm times.

Chorus functional dependencies derived from CRRES data

Using data from the Combined Release and Radiation Effects Satellite (CRRES) plasma wave experiment, we present quadratic fits to the mean of the wave amplitude squared, , for lower and upper band chorus as a function of geomagnetic activity (Kp) and magnetic latitude (λ) for the midnight (21<MLT≤3), dawn (3<MLT≤9), and postnoon (12<MLT≤15) sectors and present an ad hoc model for the prenoon (9<MLT≤12) sector. The empirical models are valid for 4≤L≤7, |λ|<15° and Kp≤6 and quantify the dramatic difference in the latitudinal dependence of chorus as a function of local time and how that dependence varies with geomagnetic condition, critical parameters for calculating wave‐induced electron scattering rates. Our results are generally consistent with previous studies, and our key contribution is the quantification of wave properties in a manner that is easily incorporated into global quasi‐linear diffusion models.

A statistical study of the inner edge of the electron plasma sheet and the net convection potential as a function of geomagnetic activity

[1] A widely accepted explanation of the location of the inner edge of the electron plasma sheet and its dependence on electron energy is based on drift motions of individual particles. The boundary is identified as the separatrix between drift trajectories linking the tail to the dayside magnetopause (open paths) and trajectories closed around the Earth. A statistical study of the inner edge of the electron plasma sheet using THEMIS Electrostatic Analyzer plasma data from November 2007 to April 2009 enabled us to examine this model. Using a dipole magnetic field and a Volland‐Stern electric field with shielding, we find that a steady state drift boundary model represents the average location of the electron plasma sheet boundary and reflects its variation with the solar wind electric field in the local time region between 21:00 and 06:00, except at high activity levels. However, the model does not reproduce the observed energy dispersion of the boundaries. We have also used the location of the inner edge of the electron plasma sheet to parameterize the potential drop of the tail convection electric field as a function of solar wind electric field (Esw) and geomagnetic activity. The range of Esw examined is small because the data were acquired near solar minimum. For the range of values tested (meaningful statistics only for Esw < 2 mV/m), reasonablygoodagreementisfoundbetweenthepotentialdropofthetailconvectionelectric field inferred from the location of the inner edge and the polar cap potential drop calculated from the model of Boyle et al. (1997).

Cosmic ray decreases and geomagnetic activity: list of events 1982–2002

Decreases in cosmic ray (CR) intensity provided by neutron monitors, for the time period 1982–2002, are summarized and compared with simultaneous changes in Dst index. Data from four neutron monitors with different vertical cutoff rigidities are used, namely Oulu (∼0.8 GV) , Lomnický Štı́t (∼4 GV) and Haleakala/Huancayo (∼13 GV) . The Dst behaviour during different periods of CR decreases is rather complex. Along with the events when CR decreases are accompanied by corresponding Dst depressions, there are several effects when geomagnetic activity (measured by Dst) is not changing significantly. In some cases large Dst depression is not marked by CR decreases. This work presents 2 tables of CR events as a function of geomagnetic activity. The tables, indicating 19% of CR decreases without Dst variability and 28% of Dst variabilities without clear CR decreases, may be useful for investigating the relation between CR flux values and atmospheric effects and for eventual discrimination between CR flux and geomagnetic activity level influence on the atmospheric processes.

An average image of proton plasma pressure and of current systems in the equatorial plane derived from AMPTE/CCE‐CHEM measurements

The present study attempts to visualize the global equatorial current systems and the proton pressure in the near‐Earth magnetosphere based on AMPTE/GCE‐CHEM measured proton distributions, which were sorted by the AE index (“quiet”: AE &lt; 100 nT, “active”: 100 nT &lt; AE &lt; 600 nT). The data were averaged over 2 years (from January 1985 to June 1987) in order to obtain the necessary spatial resolution with statistical significance. The results provide an average image of proton plasma pressure, proton plasma anisotropy, and current systems as a function of geomagnetic activity. In particular, the changes of pressure anisotropy with local time and the noon‐midnight pressure asymmetry are studied and correlated to the current systems out of the equatorial plane that generate the closure circuits. Moreover, we identify and spatially locate two different current systems for the quiet period (the ring current and the inner portion of the quiet time cross‐tail current) and three different current structures for the active period (the ring current, the partial ring current, and the region 2 current).

Statistical X ray auroral emissions – PIXIE observations

The Polar Ionospheric X ray Imaging Experiment (PIXIE) on board the NASA/GGS POLAR spacecraft has been making observations of X rays emitted simultaneously over the entire auroral zone since March, 1996. PIXIE remotely senses the bremsstrahlung produced by the loss of multi‐keV electrons from the magnetosphere into the ionosphere. In this study we examine twenty‐six months (April 1996 – July 1998) of X ray observations of the northern auroral region from the PIXIE instrument. X ray flux is obtained from the PIXIE observations, which allows us to produce for the first time synoptic auroral maps representing the precipitation of energetic electrons, as a function of geomagnetic activity.

Medium energy particle precipitation influences on the mesosphere and lower thermosphere

Data accumulated for more than a solar cycle by the TIROS/NOAA polar orbiting satellites are used to extract patterns of particle precipitation in several medium energy bands for varying geomagnetic conditions. While the atmospheric effects of auroral particles measured by these satellites (300 eV‐20 keV) have been widely studied, the effects of medium energy (30 keV‐2.5 MeV) particles have received less attention; in this paper we present average particle precipitation patterns for medium energy protons and electrons as a function of geomagnetic activity. A magnetic coordinate system is used to bin data at high latitudes to produce patterns that are then used as additional inputs to the National Center for Atmospheric Research thermosphere ionosphere mesosphere electrodynamic general circulation model (TIME‐GCM) to study the effects of medium energy particles on the mesosphere and lower thermosphere. Model results obtained with these additional inputs are used to assess the significance of medium energy particles on the composition and dynamics of the region. Zonally averaged model results from the run that includes the medium energy particles show significant increases in ne, HOX, NO2, and NO and a 27 percent decrease in O3 between 70 and 80 km in the polar regions when compared to a similar run without the particles. The temperature and wind differences are small.

Relative impact of meteor scatter and other long‐distance high‐latitude propagation modes on VHF communication systems

We have analyzed the duty cycle, due to ionospheric propagation, of very high frequency sounding signals for both polar cap and auroral paths. We find that at 35 and 45 MHz the propagation is often sustained by sporadic E layers and other nonmeteoric modes rather than by meteor scatter. At the higher frequencies of 65 and 85 MHz we find that the path is generally dominated by meteor scatter modes. These results have important ramifications for frequency reuse and security in meteor burst communications systems and for the development of extended frequency range HF systems (above 30 MHz) with a capability to operate on any available propagation mode. The diurnal, seasonal, and geomagnetic variations of the nonmeteoric duty cycle have been examined. A polar cap path model is presented for the nonmeteoric duty cycle as a function of geomagnetic activity.

Reference models for thermospheric nitric oxide, 1994

Rocket and satellite measurements of nitric oxide have been made by observing the ultraviolet airglow from this molecule. Nitric oxide has been measured with an ultraviolet spectrometer on the polar-orbiting satellite Solar Mesosphere Explorer (SME) for the period January 1982 to August 1986. The nitric oxide database contains densities at all latitudes sorted into 5° bins and at altitudes between 100 and 160 km sorted into 3.3-km bins. The largest densities occur at latitudes in the auroral zones where the density varies as a function of geomagnetic activity. At low latitudes, the nitric oxide density varies in response to changes in the solar soft x-ray and extreme ultraviolet radiation. Rocket experiments made at a mid-latitude launch site are compared to the satellite observations.

The Average Pattern of Auroral Particle Precipitation, Its Associated Conductivity and Field Aligned Currents

A series of statistical studies has been completed to determine the global pattern of auroral electron and ion precipitation and their resultant Hall and Pedersen conductivities as a function of geomagnetic activity, solar wind velocity, the orientation of the Interplanetary Magnetic Field (IMF) and season. In addition, studies were performed relating these global patterns in particle precipitation to similar global determinations of the average delta B vector produced by the auroral field aligned currents. The data for these studies were from instruments flown on the satellites of the Defense Meteorological Satellite Program (DMSP). In all cases the studies were performed by dividing the high latitude region into a series of spatial bins in Magnetic Local Time (MLT) and corrected geomagnetic latitude (CGL). One such matrix of spatial bins was created for each different value or range of values of chosen sort parameter. For geomagnetic activity the sort parameter was one of seven levels of Kp. For the IMF and solar wind velocity the sort parameters consisted of 30 paired ranges of the solar wind velocity and the Bz component of the IMF. A rough separation by the IMF By was performed by using the IMF sector structure and Kp together as the sort parameter. Seasonal separations were made with Kp and for time periods centered on the summer and winter solstices and the equinoxes. In all cases the large DMSP data sets were used to determine the average spectrum of precipitating electrons and ions and the average delta B vector for each spatial bin and for each sort parameter used. In this paper we review the results of these studies.

Reference models for thermospheric no

Nitric oxide has been measured with an ultraviolet spectrometer on the polar-orbiting satellite Solar Mesosphere Explorer (SME) for the period January 1982 to August 1986. The nitric oxide database contains densities at all latitudes sorted into 5 degree bins and at altitudes between 100 and 140 km sorted into 3.3 km-bins. The largest densities occur at latitudes in the auroral zones where the density varies as a function of geomagnetic activity. Variations of a factor of 10 occur between times of intense activity and quiet times. At low latitudes, the nitric oxide density at 110 km varies from a mean value of 3 times 10(exp 7) molecules per cubic cm in January 1982 to a mean value of 4 times 10(exp 6) molecules per cubic cm during solar minimum conditions in 1986. In addition, the low-latitude nitric oxide density varies plus or minus 50 percent with a period of 27 days during times of high solar activity.