Precipitating Energy Flux Research Articles

Mapping the electron energy in Jupiter's aurora: Hubble spectral observations

AbstractFar ultraviolet spectral observations have been made with the Hubble Space Telescope in the time‐tag mode using the Space Telescope Imaging Spectrograph (STIS) long slit. The telescope was slewed in such a way that the slit projection scanned from above the polar limb down to midlatitudes, allowing us to build up the first spectral maps of the FUV Jovian aurora. The shorter wavelengths are partly absorbed by the methane layer overlying part of the auroral emission layer. The long‐wavelength intensity directly reflects the precipitated energy flux carried by the auroral electrons. Maps of the intensity ratio of the two spectral regions have been obtained by combining spectral emissions in two wavelength ranges. They show that the amount of absorption by methane varies significantly between the different components of the aurora and inside the main emission region. Some of the polar emissions are associated with the hardest precipitation, although the auroral regions of strong electron precipitation do not necessarily coincide with the highest electron energies. Outputs from an electron transport model are used to create maps of the distribution of the characteristic electron energies. Using model atmospheres adapted to auroral conditions, we conclude that electron energies range between a few tens to several hundred keV. Comparisons of derived energies are in general agreement with those calculated from magnetosphere‐ionosphere coupling models, with values locally exceeding the standard model predictions. These results will provide useful input for three‐dimensional modeling of the distribution of particle heat sources into the high‐latitude Jovian upper atmosphere.

Dayglow and auroral emissions of Uranus in H2 FUV bands

Following the recent detection of an auroral signal on Uranus (Lamy et al. [2012]. Geophys. Res. Lett. 39, 7105) during HST/STIS observation performed in November 2011, we analyzed the associated HST/STIS FUV spectral images obtained in 2011 and 2012. Our purpose was to extract any possible H2 emission produced in the upper atmosphere of the planet. To interpret these data, we adapted a version of the kinetic Trans* code to the Uranian case. This code simulates the H2 emissions created by energetic particle precipitations in the upper atmosphere. The signal measured in the 1330–1700Å range, of around 1.8kR, is composed mostly of reflected sunlight with a small contribution from upper atmospheric emissions. For most spectra, we found no evidence of particle precipitation, indicating a precipitation flux smaller than 0.01ergcm−2s−1. For the spectrum of 29 November 2011 however, when an auroral spot was positively detected, we additionally identified a small contribution of H2 emission which requires a precipitating energy flux up to 0.05ergcm−2s−1. This study also establishes that the Hubble Space Telescope (HST) can be used to monitor the aurorae on Uranus in its image and spectral modes, and to estimate the associated precipitated energy flux, provided a very careful data processing is applied.

Evolution of the Io footprint brightness I: Far-UV observations

The Io footprint (IFP) is a set of auroral spots and an extended tail resulting from the strong interaction between Io and the Jovian magnetosphere. For the first time, we present measurements of the brightness and precipitated power for each individual spot, using the image database gathered from 1997 to 2009 with the Hubble Space Telescope in the Far-UV domain. We show that the relative brightness of the spots varies with the System III longitude of Io. Moreover, our novel measurement method based on 3D simulations of the auroral features allows to derive the precipitated energy fluxes from images on which the emission region is observed at a slant angle. Peak values as high as 2W/m2 are observed for the main spot, probably triggering a localized and sudden heating of the atmosphere. Additionally, strong brightness differences are observed from one hemisphere to another. This result indicates that the location of Io in the plasma torus is not the only parameter to control the brightness, but that the magnetic field asymmetries also play a key role. Finally, we present new data confirming that significant variations of the spots' brightness on timescales of 2–4min are ubiquitous, which suggests a relationship with intermittent double layers close to Jovian surface.

Signatures of magnetospheric injections in Saturn's aurora

We report on the evolution of auroral structures based on two 3 h sequences of Saturn's northern hemisphere obtained with the UVIS instrument on board Cassini, and we discuss their possible association with injections in the magnetosphere of Saturn. Simultaneously with the UV auroral structures, we observe Energetic Neutral Atom (ENA) enhancements which are indicative of a rotating heated plasma region possibly related to magnetospheric injections. We examine the possibility that the UV auroral structures reported here are triggered by energetic particle injections by investigating (a) the evolution of the longitudinal extent of an injection and (b) its energy density, properties that change with time due to dispersion and ion/electron losses. We simulate the auroral counterpart of an injection considering that the precipitating energy flux could be provided to the ionosphere by pitch angle diffusion and electron scattering by whistler‐mode waves. We compare the brightness and size evolution of the simulated ionospheric signature with the observed values, and we demonstrate that the UV auroral structure behaves as an auroral signature of an injection. This comparative study defines characteristics of the injections such as the spectral index and the electron energy range as well as the magnetospheric corotation rate. Additionally, based on the simultaneous ENA‐UV emissions, we discuss the possibility that pitch angle diffusion and electron scattering may not be the only mechanism responsible for the observed auroral emissions. Field‐aligned currents driven by the pressure gradients along the boundaries of the injected hot plasma cloud could be also considered to play a role on how injections create auroral emissions at Saturn.

Geomagnetic and auroral activity driven by corotating interaction regions during the declining phase of Solar Cycle 23

AbstractA superposed epoch analysis is performed to investigate the relative impact of the solar wind/interplanetary magnetic field (IMF) on geomagnetic activity, auroral hemispheric power, and auroral morphology during corotating interaction regions (CIRs) events between 2002 and 2007, when auroral images from Thermosphere Ionosphere Mesosphere Energetics and Dynamics/Global Ultraviolet Imager were available. Four categories of CIRs have been compared. These were classified by the averaged IMF Bz and the time of maximum solar wind dynamic pressure around the CIR stream interface or onset time. It is found that during CIR events: (1) The peaks of auroral power and Kp were largely associated with dominant southward Bz, whereas auroral activity also became stronger with increases of solar wind speed, density, and dynamic pressure. (2) The percentage and absolute increases of auroral hemispheric power with solar wind speed were much greater under dominantly northward Bz conditions than under dominantly southward Bz conditions. (3) The enhancement of the auroral power and Kp with increasing solar wind speed followed the same pattern, for both dominantly southward and northward Bz conditions, regardless of the behavior of solar wind density and dynamic pressure. These results suggest that, during CIR events, southward Bz played the most critical role in determining geomagnetic and auroral activity, whereas solar wind speed was the next most important contributor. The solar wind dynamic pressure was the less important factor, as compared with Bz and solar wind speed. Relatively strong auroral precipitation energy flux (> ~3 mW/m2) occurred in a wider auroral oval region after the stream interface than before it for both dominantly northward and southward Bz conditions. These conditions enhanced the auroral hemispheric power after the stream interface. Intense auroral precipitation (> ~4 mW/m2) generally occurred widely at night under dominantly southward Bz conditions, but the location of this precipitation in the auroral oval was different when it was associated with different solar wind density and speed conditions.

Longitudinal variations of nighttime electron auroral precipitation in both the Northern and Southern hemispheres from the TIMED global ultraviolet imager

Using 6 years of Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) global ultraviolet imager auroral observations in both hemispheres, we have studied the longitudinal variations of auroral precipitation during the magnetic nighttime period of 2100-0300 magnetic local time. There was a strong seasonal dependence of the longitudinal variations of the aurora: (1) During solstices and for both hemispheres, auroral precipitation peaked between magnetic longitude (MLON) 210 degrees E and 360 degrees E in June and between MLON 120 degrees E and 300 degrees E in December. (2) In the equinoxes, the auroral longitudinal pattern was generally similar to that in local summer in each hemisphere, except that in the Northern Hemisphere the maximum precipitation was usually located in more westward longitudes in equinox than in summer. (3) The ratios between the maximum and the minimum of the precipitation energy flux along longitudes varied between 1.3 and 1.9, which were similar to those in previous studies. These features of the auroral longitudinal patterns did not change much from Kp = 1 to Kp = 4 conditions. Since the longitudinal distribution of auroral precipitation changed greatly with season in each hemisphere, the longitudinal variations of the magnetic field strength, which do not change with season, might not be the only process that caused the observed longitudinal variations of the aurora. Further data analysis shows that there was a significant negative correlation (coefficient vertical bar r vertical bar = similar to 0.4-0.8) between the peak auroral precipitation intensity and the solar-EUV-produced ionospheric conductivity of the same hemisphere (in summer and equinox) or of the conjugate hemisphere (in winter). These results indicate the important effects of solar-EUV-produced ionospheric conductivity, which has significant longitudinal variations, on the longitudinal patterns of the aurora at magnetic nighttime. Our results also suggest that the interhemispheric coupling during solstices might be an important factor that contributes to the longitudinal variations of the nighttime aurora. Our correlation analysis indicates that the hemispheric differences in the conjugate magnetic field strengths also contribute to the longitudinal variations of the aurora, although they appear not to be a major factor.

Seasonal and hemispheric variations of the total auroral precipitation energy flux from TIMED/GUVI

The auroral hemispheric power (HP) has been calculated from the averaged energy flux derived from Far‐ultraviolet emission observations made by the global ultraviolet imager (GUVI) instrument on board the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite during 2002–2007. This HP was used to study how variations in seasonal and hemispheric asymmetries changed with changing geomagnetic activity. Our results showed that there were persistent seasonal and hemispheric differences in quiet conditions. There were HP differences of about 1–3 GW between the summer and winter seasons in each hemisphere and also between the two hemispheres during each solstice period for low geomagnetic activity (Kp ≤ 3). The summer‐winter asymmetry was 4%–35% when HP was low. These summer‐winter differences became negligible when geomagnetic activity was moderate to active. Similarly, there were also HP differences of about 2 GW between local summers of the two hemispheres during geomagnetically quiet conditions (Kp < 3) but not during higher Kp conditions. The hemispheric asymmetries between the two summer solstices were about 10%–20% during quiet conditions, whereas there was no apparent hemispheric asymmetry between the two winter solstices under all Kp 1–5 conditions. Solar illumination effects were probably the primary cause of the seasonal and hemispheric variations of the auroral hemispheric power for these geomagnetically quiet conditions. During moderate and active conditions the conductivities were driven more by the production of ionization due to precipitation, so the precipitation was more symmetric.

Spectral observations of FUV auroral arcs and comparison with inverted‐V precipitating electrons

This paper presents first simultaneous observations of far ultraviolet (FUV) spectra of discrete and diffuse auroras, together with precipitating electrons measured on the same spacecraft, to emphasize the importance of high‐resolution FUV images for accurate estimation of precipitating energy flux in the auroral region. An FUV spectrograph image with ∼2 km × 3 km resolution show small‐scale features were embedded in the auroral arcs. Comparison of peak energies of inverted‐V events with the corresponding FUV spectra shows that the observed long band N2 Lyman‐Birge‐Hopfield (LBH) emission (long LBH band (LBHL): 160.0–171.5 nm) varied more sensitively to the peak energies compared to the short band. Comparison of the inverted‐V structures and their energy fluxes with the LBHL irradiance for ∼10 km × 10 km regions show they are well correlated for peak energy >2 keV. When the data are averaged over a larger area (70 km × ∼140 km), on the other hand, the LBHL irradiance becomes less bright for the corresponding electron energy flux due to the contribution from the low‐intensity background diffuse aurora produced by secondary electrons. This study demonstrates a reliable relationship between precipitating electron energy flux and LBHL intensity is obtained only if the precipitating region and FUV intensity are locally matched with a scale of less than 10 km corresponding to the size of discrete auroras.

Equatorward diffuse auroral emissions at Jupiter: Simultaneous HST and Galileo observations

We study the auroral emissions equatorward of the main oval based on Hubble Space Telescope (HST) observations of both Jovian hemispheres on September 20, 1997. On the same day, Galileo observed changes in the electron pitch angle distribution between the inner and middle magnetosphere (PAD boundary), indicative of electron diffusion. This region, magnetically maps to the equatorward diffuse emissions on both hemispheres. Whistler mode waves, observed simultaneously, can scatter electrons into the loss cone and lead to electron precipitation in the ionosphere. Based on simultaneous HST FUV and Galileo wave and electron data we test the conditions for electron scattering by whistler mode waves and derive the energy flux precipitated in the ionosphere. The comparison of the derived precipitation energy flux with the observed auroral brightness indicates that the energy contained in the PAD boundary can account for the auroral emissions.

Global observations of electromagnetic and particle energy flux for an event during northern winter with southward interplanetary magnetic field

Abstract. The response of the polar ionosphere–thermosphere (I-T) system to electromagnetic (EM) energy input is fundamentally different to that from particle precipitation. To understand the I-T response to polar energy input one must know the intensities and spatial distributions of both EM and precipitation energy deposition. Moreover, since individual events typically display behavior different from statistical models, it is important to observe the global system state for specific events. We present an analysis of an event in Northern Hemisphere winter for sustained southward interplanetary magnetic field (IMF), 10 January 2002, 10:00–12:00 UT, for which excellent observations are available from the constellation of Iridium satellites, the SuperDARN radar network, and the Far-Ultraviolet (FUV) instrument on the IMAGE satellite. Using data from these assets we determine the EM and particle precipitation energy fluxes to the Northern Hemisphere poleward of 60° MLAT and examine their spatial distributions and intensities. The accuracy of the global estimates are assessed quantitatively using comparisons with in-situ observations by DMSP along two orbit planes. While the location of EM power input evaluated from Iridium and SuperDARN data is in good agreement with DMSP, the magnitude estimated from DMSP observations is approximately four times larger. Corrected for this underestimate, the total EM power input to the Northern Hemisphere is 188 GW. Comparison of IMAGE FUV-derived distributions of the particle energy flux with DMSP plasma data indicates that the IMAGE FUV results similarly locate the precipitation accurately while underestimating the precipitation input somewhat. The total particle input is estimated to be 20 GW, nearly a factor of ten lower than the EM input. We therefore expect the thermosphere response to be determined primarily by the EM input even under winter conditions, and accurate assessment of the EM energy input is therefore key to achieving a comprehensive understanding of the I-T system, particularly during active times when the energy input increases markedly and expands well equatorward of nominal auroral latitudes.

Discontinuity in Jupiter's main auroral oval

On the basis of a series of FUV Hubble Space Telescope images obtained between 1997 and 2007 it is shown that there is a segment of the main auroral oval where the emission drops significantly from a few hundreds to a few tens of kiloRayleigh, forming a discontinuity in the oval. It is shown that the discontinuity is present in both hemispheres and confined in magnetic local time. Its equatorial source is located in the prenoon and early noon sector. The main auroral oval is associated with the ionosphere‐magnetosphere coupling current system which is related to the breakdown of corotation in the middle magnetosphere. Necessary for the electron precipitation in the ionosphere and the formation of the main auroral oval is the presence of upward field‐aligned currents, carried by downward moving electrons. Field‐aligned currents inferred by Pioneer, Voyager and Galileo in situ observations in the near equatorial plane showed evidence of reduced or/and downward field‐aligned currents in the prenoon and early afternoon sector, the location of the equatorial source of the discontinuity. Additionally, we estimate the precipitation energy flux in the ionosphere, for a typical reduced upward field‐aligned current value at that region, which is found to be within the range of the observed brightness of the discontinuity. Field aligned current distributions in the ionosphere based on magnetohydrodynamic simulations of the interaction between the solar wind and the magnetosphere have predicted a region of downward currents implying a discontinuity at the main auroral oval emission, in very good agreement with the HST observations presented in this work.

A quantitative comparison of lightning‐induced electron precipitation and VLF signal perturbations

VLF signal perturbations recorded on the Holographic Array for Ionospheric/Lightning Research (HAIL) are quantitatively related to a comprehensive model of lightning‐induced electron precipitation (LEP) events. The model consists of three major components: a test‐particle model of gyroresonant whistler‐induced electron precipitation, a Monte Carlo simulation of energy deposition into the ionosphere, and a model of VLF subionospheric signal propagation. For the two representative LEP events studied, the model calculates peak VLF amplitude perturbations within a factor of three of those observed, well within the expected variability of radiation belt flux levels. The phase response of the observed VLF signal to precipitation varied dramatically over the course of the two nights and this variability in phase response is not properly reproduced by the model. The model calculates a peak in the precipitation that is poleward displaced ∼6° from the causative lightning flash, consistent with observations. The modeled precipitated energy flux (E > 45 keV) peaks at ∼1 × 10−2 (ergs s−1 cm−2), resulting in a peak loss of ∼0.001% from a single flux tube at L ∼ 2.2, consistent with previous satellite measurements of LEP events. The precipitation calculated by the model is highly dependent on the near‐loss‐cone trapped radiation belt flux levels assumed, and hence our main objective is not to compare the model calculations and the VLF signal observations on an absolute basis but is rather to develop metrics with which we can characterize the VLF signal perturbations recorded on HAIL in terms of the associated precipitation flux. Metrics quantifying the ionospheric density enhancement (NILDE) and the electron precipitation (Γ) along a VLF signal path are strongly correlated with the VLF signal perturbations calculated by the model. A conversion ratio Ψ, relating VLF signal amplitude perturbations (ΔA) to the time‐integrated precipitation (100–300 keV) along the VLF path (Ψ = Γ/ΔA), of 1.2 ± 0.3 × 1010 (el m−1/dB) is suggested for precipitation events of similar location and characteristics to those examined. The total precipitation (100–300 keV) induced by one of the representative LEP events is estimated at ∼1.8 ± 0.4 × 1016 electrons, calculated directly from the conversion ratio Ψ and observations of VLF signal perturbations.

The Venus ultraviolet oxygen dayglow and aurora: Model comparison with observations

We compare the intensity of the OI 130.4 and 135.6 nm emissions calculated using the soft electron precipitation measured on board the Pioneer Venus (PV) Orbiter with the auroral brightness observed with the ultraviolet spectrometer (OUVS) on board the PV. For this purpose, we use a new electron transport model based on a Monte Carlo implementation of the Boltzmann equation and a multi-stream radiative transfer model to calculate the effects of multiple scattering on the intensity field of the 130.4-nm triplet. We show that the consideration of the enhancement of the emergent 130.4-nm to the 135.6-nm intensity by multiple scattering in the optically thick Venus atmosphere increases the auroral 130.4/135.6 ratio by a factor of about 3. We find agreement with the mean 130.4/135.6 ratio observed with PV-OUVS using the typical suprathermal electron energy spectrum reported from PV in situ measurements showing a characteristic energy of about 14 eV. To account for the average OI auroral emissions, the required precipitated energy flux is 2×10 −3 mW m −2, that is about 30% of the measured suprathermal night-side soft electron spectrum used as a reference. The calculated brightness of the CO Cameron bands is about twice as large as the weak observed emission, but within the error bars of the observations and the uncertainties of the dissociative excitation cross-section of CO 2. The electron transport model, coupled with calculations of excitation processes is also applied to an analysis of the FUV oxygen day airglow observations made with PV-OUVS and the Hopkins Ultraviolet Telescope (HUT) spectrograph. Comparisons indicate that the model accounts for both the disc-averaged intensities observed with the HUT spectrograph, the limb scans and the 130.4-nm images obtained with PV-OUVS. The relative contribution of resonance scattering of the solar line and photoelectron impact to the excitation of the 130.4-nm triplet depends on the altitude, but is globally dominated by resonance scattering. The intensity of the 130.4-nm dayglow emission does not vary proportionally with the O density in the lower thermosphere, but provides nevertheless a useful tool to remotely probe the atomic oxygen density and its variations.

Formula representation of auroral ionospheric O+ outflows based on systematic simulations with effects of soft electron precipitation and transverse ion heating

Systematic Dynamic Fluid Kinetic (DyFK) model simulations are conducted to obtain a simulation‐based formula representation of the O+ outflow flux level versus the energy flux of soft electron precipitation, and wave spectral density of transverse wave heating in the high‐latitude auroral region. Based on results of 140 DyFK simulations of auroral outflows, we depict the O+ outflows at the ends of these two hour simulation runs in spectrogram form versus these parameters. We also approximate the results by the formula representation: = 8.8(3.0 × 105 + 2fe1.4 × 107)(tanh (8Dwave) + 0.2Dwave0.6), where is the O+ number flux in cm−2/s at 3 RE mapped to 1000 km altitude, fe is the electron precipitation energy flux in ergs cm−2/s, and Dwave is the wave electric field spectral density at O+ gyrofrequency at 1 RE in (mV)2 m−2 Hz−1. This formula representation provides a convenient way to set the boundary conditions of the ionosphere in global magnetosphere simulation since the ionosphere is the important plasma source of the magnetosphere.

Subionospheric VLF observations of transmitter‐induced precipitation of inner radiation belt electrons

Ionospheric effects of energetic electron precipitation induced by controlled injection of VLF signals from a ground based transmitter are observed via subionospheric VLF remote sensing. The 21.4 kHz NPM transmitter in Lualualei, Hawaii is keyed ON‐OFF in 30 minute periodic sequences. The same periodicity is observed in the amplitude and phase of the sub ionospherically propagating signals of the 24.8 kHz NLK (Jim Creek, Washington) and 25.2 kHz NLM (LaMoure, North Dakota) transmitters measured at Midway Island. Periodic perturbations of the NLK signal observed at Palmer, Antarctica suggest that energetic electrons scattered at longitudes of NPM continue to be precipitated into the atmosphere as they drift toward the South Atlantic Anomaly. Utilizing a model of the magnetospheric wave‐particle interaction, ionospheric energy deposition, and subionospheric VLF propagation, the precipitated energy flux induced by the NPM transmitter is estimated to peak at L ∼ 2 and ∼ 1.6 × 10−4 ergs s−1 cm−2.

Temporal signatures of radiation belt electron precipitation induced by lightning‐generated MR whistler waves: 1. Methodology

We present a novel technique designed to calculate the detailed differential number flux signature (as a function of energy and time) of precipitating radiation‐belt electrons, driven by a magnetospherically reflecting (MR) whistler wave, initiated by a single cloud‐to‐ground lightning discharge. Our model consists of several stages. First, we calculate the MR whistler wave characteristics at 1° latitude intervals along a given field‐line. This is accomplished using an extensive ray tracing and interpolation algorithm involving ∼120 million rays, and accounting for the effects of Landau damping, spatial, and temporal dispersion. We then use these wave characteristics to compute the pitch angle changes of resonant electrons by assuming that the interactions are linear, and independent between adjacent latitude and wave frequency bins. The pitch angle changes are transformed to precipitating flux using a novel convolution method and displayed as a function of particle energy and time at the feet of a given field line. We have calculated and compared the differential number flux signatures at the northern and southern feet of the L = 2.3 and L = 3 field lines, and found that precipitation onset and duration times increase with latitude (consistent with previous work). The precipitation consists of suprathermal Landau resonance electrons (E ≲ 10 keV) which are intense but contribute little to the total energy flux, a flux gap (10 keV ≲ E ≲ 80 keV) corresponding to a change in coupling mechanism from Landau resonance to gyroresonance, and a series of precipitation swaths (E ≳ 80 keV) corresponding to gyroresonance interactions. The swaths result in periodic maxima in the precipitated energy flux, which correspond to the equatorial traversals of the underlying MR whistler wave energy. Global precipitation signatures were computed for a number of lightning discharge latitudes and are presented in a companion paper (Bortnik et al., 2006).

Temporal signatures of radiation belt electron precipitation induced by lightning‐generated MR whistler waves: 2. Global signatures

We use the methodology presented in a companion paper to calculate the temporal and spatial precipitation signatures of energetic radiation‐belt electrons due to pitch‐angle scattering by magnetospherically reflecting (MR) whistler waves generated by lightning discharges at geomagnetic source latitudes of λs = 25°, 35°, 45°, and 55°. We show precipitated energy and number fluxes, as well as average precipitated energy as a function of time and L‐shell for all four λs cases, and then extrapolate the results of the λs = 35° case in longitude to produce a time‐sequence of geographic ‘hot spots’ which are affected by the MR whistler induced electron precipitation. We then discuss the total precipitation in both hemispheres due to the various λs cases. Our major findings are that the precipitation region moves to higher L‐shells as a function of time, on short (0.1 sec, at the start of the event) and long (10 sec) timescales, corresponding to the first hop of the wave, and the MR portion of the whistler wave, respectively. There is also structure within the long‐timescale precipitation on the order of ∼1 sec, reflecting the periodic MR of the underlying whistler wave. As λs increases, an additional precipitated flux signature which is more incoherent and discontinuous, begins to form at higher L‐shells and later times, due to MR whistler wave reflections from the plasmapause. At lower L‐shells, a pronounced maximum occurs in the number flux of ∼1 keV electrons at L ∼ 2–3 due to the Landau resonance. The geographic hot spot affected by the precipitation can split into two separate regions per hemisphere, and occur simultaneously in both hemispheres so that up to four distinct precipitation hot spots can occur on the Earth at any instant, driven by a single lightning discharge. We also discuss potential sources of error, and comparison to related modeling efforts, and to observations using a either satellite‐borne instruments or ground‐based techniques.

Lightning driven inner radiation belt energy deposition into the atmosphere: regional and global estimates

Abstract. In this study we examine energetic electron precipitation fluxes driven by lightning, in order to determine the global distribution of energy deposited into the middle atmosphere. Previous studies using lightning-driven precipitation burst rates have estimated losses from the inner radiation belts. In order to confirm the reliability of those rates and the validity of the conclusions drawn from those studies, we have analyzed New Zealand data to test our global understanding of troposphere to magnetosphere coupling. We examine about 10000h of AbsPAL recordings made from 17 April 2003 through to 26 June 2004, and analyze subionospheric very-low frequency (VLF) perturbations observed on transmissions from VLF transmitters in Hawaii (NPM) and western Australia (NWC). These observations are compared with those previously reported from the Antarctic Peninsula. The perturbation rates observed in the New Zealand data are consistent with those predicted from the global distribution of the lightning sources, once the different experimental configurations are taken into account. Using lightning current distributions rather than VLF perturbation observations we revise previous estimates of typical precipitation bursts at L~2.3 to a mean precipitation energy flux of ~1×10-3 ergs cm-2s-1. The precipitation of energetic electrons by these bursts in the range L=1.9-3.5 will lead to a mean rate of energy deposited into the atmosphere of 3×10-4 ergs cm-2min-1, spatially varying from a low of zero above some ocean regions to highs of ~3-6×10-3 ergs cm-2min-1 above North America and its conjugate region.

Spectral properties and source regions of dayside electron acceleration events

Eleven years of DMSP particle data (1984–1994) were used to investigate electron acceleration events on the dayside. Specifically, the characteristic energy (spectral flux peak), total energy flux, and probability of observing acceleration events were studied as a function of the type of dayside precipitation region (cusp, mantle, etc.). Most dayside electron acceleration events are latitudinally smaller than the events found premidnight, so it is usually possible to identify the type of precipitation which surrounds dayside events. All dayside regions contain embedded electron acceleration events, with such instances most common in the dayside boundary plasma sheet (BPS) and least common in the mantle and polar rain. Electron acceleration events in the cusp are not particularly rare, but the acceleration involved is quite mild. This is consistent with findings that large nightside potentials do not form in regions with a large ambient electron population. On the basis of the fraction of spectra which are accelerated (number of accelerated spectra divided by the total number of spectra measured), the dayside regions can be ordered as follows, with the acceleration probability in parenthesis: mantle prenoon (2.4%), mantle postnoon (3.0%), cusp (4.2%), open low‐latitude boundary layer (LLBL) (5.0%), closed LLBL prenoon (6.0%), closed LLBL postnoon (7.2%), BPS prenoon (8.2%), and finally BPS postnoon (8.6%). The total precipitating energy flux from electron acceleration has the same order; thus more dayside auroral luminosity is associated with the BPS than with any other region. When these regions are ordered by the spectral flux peak in acceleration events, the order changes, as follows: cusp (201 eV), open LLBL (234 eV), closed LLBL prenoon (390 eV), mantle prenoon (414 eV), closed LLBL postnoon (459 eV), mantle prenoon (463 eV), BPS prenoon (572 eV), and BPS postnoon (725 eV). The afternoon hot spot is found to have a relatively symmetric partner, a morningside “warm spot,” centered near 0830 MLT. Both the afternoon hot spot and the morning warm spot span the convection reversal boundary, since they include both BPS and LLBL precipitation, with the former region showing the most intense activity (both the highest number of events and the largest electron acceleration energies).

Ion aurora and its seasonal variations

Recent studies have shown that intense discrete aurora, auroral kilometric radiation, upflowing ion beams, and downward directed electric fields are more intense in the winter hemisphere than in the summer. This is particularly true of the dusk to midnight sector where intense electron aurora are most common. Here we use one solar cycle of DMSP satellite particle data to investigate the seasonality of the ion aurora. The ion aurora proves to be approximately equal in the summer and winter hemispheres in the dusk‐midnight sector (with the summer hemisphere favored by 0 to 4%). However, in the MLT hours from midnight to dawn, the ion precipitating energy flux is 15–40% higher in winter than in summer. The absolute magnitude of the ion effect is smaller than was found for discrete electron aurora (which show a threefold difference between winter and summer). The seasonal behavior of the ions may reflect the observation that diverging electric fields, which accelerate ions downward, are found mainly postmidnight, and are stronger in the winter. The relative weakness of the seasonal effects in ions may reflect their high average energy (many tens of keV), which is substantially larger than typical of electric potentials found in the auroral circuit. Ions in the dusk to midnight sector are most intense equatorward of the region of discrete aurora, and are thus probably not much affected by the seasonality of the field‐aligned electric fields which exist there (and which are of the sense to retard ion precipitation). Interestingly, ion average energies are higher in the winter hemisphere than in the summer hemisphere at all local times, regardless of whether or not energy fluxes are enhanced. The above results apply to the actual range of ion energies measured by the DMSP electrostatic analyzers (32 eV to 30 keV). A sample extrapolation to 100 keV energy, assuming the spectra are Maxwellian, showed the same overall pattern. The percentage by which the winter hemisphere was favored over the summer for precipitating ion energy flux was moderately larger when using extrapolation, and a larger difference was still seen postmidnight than premidnight.