Proton Precipitation Research Articles

NOAA POES and MetOp particle observations during the 17 March 2013 storm

Five National Oceanic and Atmospheric Administration and Meteorological Operational polar orbiting low altitude satellites were in operation during the 17 March 2013 storm. They were equipped with identical instruments to measure the precipitation of energetic particles over a large energy range. Well calibrated particle data, gives the opportunity of complete ILAT/MLT plots on a time resolution of 3 h throughout the storm. During the storm sudden commencement and main phase of the storm, an isotropic proton precipitation occurred at almost all magnetic local time and the Isotropic Boundary (IB) coincided with the inner boundary of the Ring Current (RC) as determined by the Van Allen Probes. Thus the IB on the night/dusk sector is a measure of how deep the ring current develops in the inner magnetosphere. The electron and proton precipitation is closely connected with substorms and the convective electric field created by the solar wind. A loss cone totally filled with particles indicates the occurrence of particles injection into the RC, while a partly filled loss cone means that the RC particles experienced a loss.The above 1 MeV electrons disappear in the storm main phase, but appears with increased intensity in the recovery phase. They are limited to the invariant latitude region 50∘ to 62∘. The lower energy electrons penetrate deeper into the magnetosphere. During geomagnetic storms a Storm Time Equatorial Belt (STEB) of energetic neutral atoms and ions is found to exist at low altitudes around the geomagnetic equator. Their source is the RC protons existing at larger L-values. The STEB appears first in the midnight/evening sector and then later in the morning sector largely consistent with the drift of the RC ions and in accordance with results from ground based magnetic observations. During the main phase of the storm, the outer part of the RC will experience magnetopause shadowing and will be lost at the magnetopause. Maximum particle injection into the ring current is associated with the most rapid fall in Dst. The maximum STEB intensity is associated with minimum Dst and strongest RC.

Global distribution of energetic proton precipitations equatorward of the boundary of isotropic fluxes

Based on data of the NOAA POES satellite, the global distribution of the occurrence rate of precipitations of energetic protons (E > 30 keV) equatorward of the boundary of isotropic fluxes has been constructed for the first time. It has been shown that the occurrence rate of proton precipitations inside the zone of anisotropic fluxes is maximum in daytime hours (1100–1600 MLT) at latitudes L = 6–9 and decreases in evening and morning hours. Comparison of the obtained results about proton precipitations with the spatial distribution of the occurrence rate of electromagnetic ion–cyclotron (EMIC) waves in the equatorial magnetosphere according to results of satellite observations demonstrates a close relationship between them. This corroborates that precipitations of energetic protons equatorward of the boundary of isotropic fluxes are a consequence of the development of the ion–cyclotron instability in the equatorial magnetosphere.

Conjugate observations of electromagnetic ion cyclotron waves associated with traveling convection vortex events

AbstractWe report on simultaneous observations of electromagnetic ion cyclotron (EMIC) waves associated with traveling convection vortex (TCV) events caused by transient solar wind dynamic pressure (Pd) impulse events. The Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft located near the magnetopause observed radial fluctuations of the magnetopause, and the GOES spacecraft measured sudden compressions of the magnetosphere in response to sudden increases in Pd. During the transient events, EMIC waves were observed by interhemispheric conjugate ground‐based magnetometer arrays as well as the GOES spacecraft. The spectral structures of the waves appear to be well correlated with the fluctuating motion of the magnetopause, showing compression‐associated wave generation. In addition, the wave features are remarkably similar in conjugate hemispheres in terms of bandwidth, quasiperiodic wave power modulation, and polarization. Proton precipitation was also observed by the DMSP spacecraft during the wave events, from which the wave source region is estimated to be 72°–74° in magnetic latitude, consistent with the TCV center. The confluence of space‐borne and ground instruments including the interhemispheric, high‐latitude, fluxgate/induction coil magnetometer array allows us to constrain the EMIC source region while also confirming the relationship between EMIC waves and the TCV current system.

Thunderstorm effects according to data from a comprehensive study of variations in secondary cosmic ray particles

Thunderstorms have been studied at the Baksan Neutrino Observatory (Institute for Nuclear Research, Russian Academy of Sciences) since the summer of 2013 using a measuring instrumentation complex. During the period of observation, three events of continuous night-sky airglow correlating with the disturbances in the flux of energetic particles have been recorded during thunderstorms. It is established that there is interaction between a thunderstorm’s front and the precipitation of protons from the Earth’s radiation belt into the atmosphere due to seismic activity.

A dedicated H‐beta meridian scanning photometer for proton aurora measurement

AbstractAn instrument designed to measure the location and brightness of auroral emissions from energetic proton precipitation is described. This photometer scans from the north to south horizon with a stepper motor and mirror. The scans are configured in software for a 30 s cadence with equally spaced samples along a meridian at constant altitude. Broadband light is separated into two channels with a novel optical splitter. This splitter uses a filter that has high transmission for the signal channel and high reflection on both the long‐ and short‐wavelength sides to reflect the combined background passbands, directing each channel to its respective detector. The half‐cone angle and angle of incidence of this splitter filter allow for an overall compact optical design that also provides superior sensitivity in both signal and background channels. The signal channel is 3 nm wide full width at half maximum (FWHM) at 486.1 nm, and the background channel comprises two 3 nm wide FWHM passbands at 480 nm and 495 nm created by a single filter. Both of these channels are measured with photomultiplier tubes in photon‐counting mode. Calibrations indicate a response of around 1000 c/s per rayleigh. Data are currently acquired in 5 ms bins with a Nyquist frequency of 100 Hz. The first system (Forty‐Eight Sixty‐One (FESO)‐1) has been operating at Athabasca University since February 2014, and the second system (FESO‐2) was deployed at Lucky Lake, Saskatchewan, in October 2015. The improved sensitivity over legacy instruments and the simultaneous measurement of signal and background enable operation during intervals with dynamic electron aurora and scattered moonlight.

Spectra of keV protons related to ion-cyclotron wave packets

We use the Fokker-Planck-Kolmogorov equation to study the statistical aspects of stochastic dynamics of the radiation belt (RB) protons driven by nonlinear electromagnetic ion-cyclotron (EMIC) wave packets. We obtain the spectra of keV protons scattered by these waves that show steeping near the gyroresonance, the signature of resonant wave-particle interaction that cannot be described by a simple power law. The most likely mechanism for proton precipitation events in RBs is shown to be nonlinear wave-particle interaction, namely, the scattering of RB protons into the loss cone by EMIC waves.

Inferring electromagnetic ion cyclotron wave intensity from low altitude POES proton flux measurements: A detailed case study with conjugate Van Allen Probes observations

Electromagnetic ion cyclotron (EMIC) waves play an important role in the magnetospheric particle dynamics and can lead to resonant pitch-angle scattering and ultimate precipitation of ring current protons. Commonly, the statistics of in situ EMIC wave measurements is adopted for quantitative investigation of wave-particle interaction processes, which however becomes questionable for detailed case studies especially during geomagnetic storms and substorms. Here we establish a novel technique to infer EMIC wave amplitudes from low-altitude proton measurements onboard the Polar Operational Environmental Satellites (POES). The detailed procedure is elaborated regarding how to infer the EMIC wave intensity for one specific time point. We then test the technique with a case study comparing the inferred root-mean-square (RMS) EMIC wave amplitude with the conjugate Van Allen Probes EMFISIS wave measurements. Our results suggest that the developed technique can reasonably estimate EMIC wave intensities from low-altitude POES proton flux data, thereby providing a useful tool to construct a data-based, near-real-time, dynamic model of the global distribution of EMIC waves once the proton flux measurements from multiple POES satellites are available for any specific time period.

Observing the magnetosphere through global auroral imaging: 1. Observables

AbstractOver the years, it has become clear that there are fundamental limitations in observing magnetospheric processes through their auroral footprints. Most electron auroras are formed in the auroral acceleration region relatively close to the Earth at altitudes (<2 RE). There are four distinct auroral types: (1) downward field‐aligned current (FAC) regions where ion precipitation is dominant, (2) pitch angle diffusion aurora (or briefly “diffusion aurora”) region without significant FAC, (3) upward FAC regions of precipitating electrons and monoenergetic auroral arc formations, and (4) Alfvénic auroral regions, where low‐energy electrons from the ionosphere are accelerated by incoming Alfvén waves. Alfvénic auroras are the footprints of magnetospheric regions where waves are produced by dynamic events such as reconnection, substorm onset initiation, and magnetic field dipolarization. Based on the mean energy and density of the precipitating electrons, ground‐based and spacecraft‐based optical observations can be used to distinguish between auroras where the source is the plasma sheet (types 1, 2, and 3) and Alfvénic auroras, where the source is the ionosphere (type 4). Imaging of the Alfvénic auroral region could be used to map the dynamically active regions of the magnetosphere. The energy distribution of the most significant precipitating ions, protons, can be measured from the Doppler profile of the hydrogen emission lines. Mapping of the time dependent global energy distribution of proton precipitation would allow the observation of the associated magnetospheric boundaries.

Relativistic electron precipitation as seen by NOAA POES

AbstractWe performed a survey of relativistic electron precipitation (REP) events revealed by the Medium Energy Proton and Electron Detector instrument on board NOAA Polar‐orbiting Operational Environmental Satellites during a 38 day interval. We have divided the observed REP events into three groups with respect to the simultaneous observations of energetic (>30 keV) electron and proton precipitation. The first group consists of REP enhancements forming the isotropy zone at the poleward edge of trapped relativistic electron fluxes. These REP events are observed on the nightside, and they are, apparently, produced by isotropization process related to nonadiabatic motion of particles in the stretched magnetic field. The second group are the REP events related to simultaneous enhancements of energetic >30–300 keV electrons. These events have a wider magnetic local time range of occurrence with a maximum in the premidnight sector. They can be related to the interaction of electrons with waves whose possible nature is briefly discussed on the basis of comparison with the cold plasma density in the conjugated region of the equatorial plane. The third group consists of the REP events correlated with the burst‐like precipitation of >30–keV protons within an anisotropy zone, where the trapped flux dominates. These events are found in the dusk sector in association with enhanced cold plasma density in the conjugate equatorial magnetosphere. As is known, proton bursts within the anisotropy zone indicate the location of the electromagnetic ion cyclotron (EMIC) wave source. Such REP events can be due to scattering of the relativistic electrons by EMIC waves. However, we noted that some of these REP events are associated with precipitation of energetic electrons with low‐energy cutoff below 100 keV. We suggest that in such cases the electrons within a wide energy range are precipitated by other waves (probably, by plasmaspheric hiss).

Impacts of spontaneous hot flow anomalies on the magnetosheath and magnetopause

AbstractSpacecraft observations and global hybrid (kinetic ions and fluid electrons) simulations have demonstrated that ion dissipation processes at the quasi‐parallel bow shock are associated with the formation of structures called spontaneous hot flow anomalies (SHFAs). Previous simulations and recent spacecraft observations have also established that SHFAs result in the formation of magnetosheath filamentary structures (MFS). In this paper we demonstrate that in addition to MFS, SHFAs also result in the formation of magnetosheath cavities that are associated with decreases in density, velocity, and magnetic field and enhancements in temperature. We use the results of a global MHD run to determine the change in the magnetosheath properties associated with cavities due to ion kinetic effects. The results also show the formation of regions of high flow speed called magnetosheath jets whose properties as a function of solar wind Mach number are described in this study. Comparing the properties of the simulated magnetosheath cavities and jets to past spacecraft observations provides good agreement in both cases. We also demonstrate that pressure variations associated with cavities and SHFAs in the sheath result in a continuous sunward and antisunward magnetopause motion. This result is consistent with previous suggestions that SHFAs may be responsible for the generation of ion cyclotron waves and precipitation of ring current protons in the outer magnetosphere.

Resonant scattering of central plasma sheet protons by multiband EMIC waves and resultant proton loss timescales

AbstractThis is a companion study to Liang et al. (2014) which reported a “reversed” energy‐latitude dispersion pattern of ion precipitation in that the lower energy ion precipitation extends to lower latitudes than the higher‐energy ion precipitation. Electromagnetic ion cyclotron (EMIC) waves in the central plasma sheet (CPS) have been suggested to account for this reversed‐type ion precipitation. To further investigate the association, we perform a comprehensive study of pitch angle diffusion rates induced by EMIC wave and the resultant proton loss timescales at L = 8–12 around the midnight. Comparing the proton scattering rates in the Earth's dipole field and a more realistic quiet time geomagnetic field constructed from the Tsyganenko 2001 (T01) model, we find that use of a realistic, nondipolar magnetic field model not only decreases the minimum resonant energies of CPS protons but also considerably decreases the limit of strong diffusion and changes the proton pitch angle diffusion rates. Adoption of the T01 model increases EMIC wave diffusion rates at > ~ 60° equatorial pitch angles but decreases them at small equatorial pitch angles. Pitch angle scattering coefficients of 1–10 keV protons due to H+ band EMIC waves can exceed the strong diffusion rate for both geomagnetic field models. While He+ and O+ band EMIC waves can only scatter tens of keV protons efficiently to cause a fully filled loss cone at L > 10, in the T01 magnetic field they can also cause efficient scattering of ~ keV protons in the strong diffusion limit at L > 10. The resultant proton loss timescales by EMIC waves with a nominal amplitude of 0.2 nT vary from a few hours to several days, depending on the wave band and L shell. Overall, the results demonstrate that H+ band EMIC waves, once present, can act as a major contributor to the scattering loss of a few keV protons at lower L shells in the CPS, accounting for the reversed energy‐latitude dispersion pattern of proton precipitation at low energies (~ keV) on the nightside. The pitch angle coverage for H+ band EMIC wave resonant scattering strongly depends on proton energy, L shell, and field model. He+ and O+ band EMIC waves tend to cause efficient scattering loss of protons at higher energies, thereby importantly contributing to the isotropic distribution of higher energy (> ~ 10 keV) protons at higher L shells on the nightside where the geomagnetic field line is highly stretched. Our results also suggest that scattering by H+ band EMIC waves may significantly contribute to the formation of the reversed‐type CPS proton precipitation on the dawnside where both the wave activity and occurrence probability is statistically high.

Sporadic Geomagnetic Pulsations at Frequencies of up to 15 HZ in the Magnetic Storm of November 7–14, 2004: Features of the Amplitude and Polarization Spectra and their Connection with Ion–Cyclotron Waves in the Magnetosphere

We study the dynamics of the geomagnetic-pulsation spectra at unusually high frequencies (including the frequencies exceeding the Schumann resonance frequency 8 Hz), which were detected for the first time at the Novaya Zhizn’ midlatitude station (the McIlwain parameter L = 2.6) at the time of a strong magnetic storm on November 07–14, 2004. To interpret the observed pulsation frequencies, we used the data from the NOAA low-orbit satellites which recorded localized precipitations of energetic protons (with energies of 30 to 80 keV) and calculations of the singlepass cyclotron amplification of electromagnetic ion–cyclotron waves. Amplitude and polarization characteristics of the radiation spectra at frequencies of up to 15 Hz at the Novaya Zhizn’ and Lovozero stations (L = 5.2) are compared. It is shown that the magnetic field oscillations in the frequency range 7–15 Hz correlate with proton precipitations and proton auroras at geomagnetic latitudes 50°–57° (L = 2.42–3.37). It is also shown that for a high anisotropy of the pitch-angle distribution of the ring-current protons at such low geomagnetic latitudes, the frequency spectrum of observed high-frequency radiation agrees well with the calculated location of the maximum of the single-pass cyclotron amplification of electromagnetic ion–cyclotron waves. Analysis of the data and calculation results has led to the conclusion that inherently the recorded signals are a high-frequency counterpart of the Pc1 pulsations and are due to the generation of ion–cyclotron waves in the magnetosphere at unusually low latitudes, which are probably stipulated by the shift of the plasma pause to these latitudes during a strong magnetic storm.

Influence of X-ray and polar cap absorptions on vertical and oblique sounding ionograms on different latitudes

High frequency (HF) radio band is important for the long-range communications and over-the-horizon surveillance, particularly in the polar cap region where the ground infrastructure may be limited. However, the space weather events drastically affect the high frequency radio wave propagation so that the ability to provide now-casting and forecasting of HF radio wave absorption is important for users relying on the HF communications. During the space weather events such as solar proton events and X-ray flares the high-latitude ionosphere becomes a particularly efficient absorber of HF radio waves. There is therefore a need to develop accurate HF propagation prediction services. Absorption of the HF field caused by the X-ray flux, Solar Ultra-Violet flux and proton precipitations is investigated in this paper for the event of the solar flare observed on 11 April 2013. The effects of the X-ray flux and proton precipitations on the structure of the vertical and oblique ionograms for different latitudes are estimated. The simulation of the vertical and oblique ionograms was performed on the basis of the computational model of the ionosphere oriented to applications into the high frequency wave propagation problems. The absorption effects induced by the proton precipitations and X-ray flux are calculated according to the algorithm elaborated by Sauer and Wilkinson and D-region Absorption Model (D-RAP) available from the NOAA Space Weather Prediction Center. The simulated vertical and oblique ionograms with the absorption effects taken into account and the measured ionograms exhibit a fairly good similarity.

An extensive survey of dayside diffuse aurora based on optical observations at Yellow River Station

AbstractBy using 7 years optical auroral observations obtained at Yellow River Station (magnetic latitude 76.24°N) at Ny‐Alesund, Svalbard, we performed the first extensive survey for the dayside diffuse auroras (DDAs) and acquired observational results as follows. (1) The DDAs can be classified into two broad categories, i.e., unstructured and structured DDAs. The unstructured DDAs are mainly distributed in morning and afternoon, but the structured DDAs predominantly occurred around the magnetic local noon (MLN). (2) The unstructured DDAs observed in morning and afternoon present obviously different properties. The afternoon ones are much stable and seldom show pulsating property. (3) The DDAs are more easily observed under geomagnetically quiet times. (4) The structured DDAs mainly show patchy, stripy, and irregular forms and are often pulsating and drifting. The drifting directions are mostly westward (with speed ~5 km/s), but there are cases showing eastward or poleward drifting. (5) The stripy DDAs are exclusively observed near the MLN and, most importantly, their alignments are confirmed to be consistent with the direction of ionospheric convection near the MLN. (6) A new auroral form, called throat aurora, is found to be developed from the stripy DDAs. Based on the observational results and previous studies, we proposed our explanations to the DDAs. We suggest that the unstructured DDAs observed in the morning are extensions of the nightside diffuse aurora to the dayside, but that observed in the afternoon are predominantly caused by proton precipitations. The structured DDAs occurred near the MLN are caused by interactions of cold plasma structures, which are supposed to be originated from the ionospheric outflows or plasmaspheric drainage plumes, with hot electrons from the plasma sheet. We suppose that the cold plasma structures for producing the patchy DDAs are in lumpy and are more likely from the plasmaspheric drainage plumes. The cold plasma structure for producing the stripy DDAs should be in wedge like and is generated by conveying the cold plasmas from lower L‐shell toward higher L‐shell with magnetospheric convection, and that for producing the irregular DDAs is resulted from deforming the wedge‐like structure by disturbance. The throat aurora is supposed to be projection of a newly opened flux of reconnection. In addition, we also found that structured DDAs correspond to structured electron precipitations in the ionosphere, which implies that the cold plasma structures in the magnetosphere are magnetically mapped to the ionosphere and act as a duct for producing the structured DDAs. We argue that we have presented some new observational results about DDA in this paper, which will be useful for fully understanding the DDAs.

Van Allen probes, NOAA, GOES, and ground observations of an intense EMIC wave event extending over 12 h in magnetic local time

AbstractAlthough most studies of the effects of electromagnetic ion cyclotron (EMIC) waves on Earth's outer radiation belt have focused on events in the afternoon sector in the outer plasmasphere or plume region, strong magnetospheric compressions provide an additional stimulus for EMIC wave generation across a large range of local times and L shells. We present here observations of the effects of a wave event on 23 February 2014 that extended over 8 h in UT and over 12 h in local time, stimulated by a gradual 4 h rise and subsequent sharp increases in solar wind pressure. Large‐amplitude linearly polarized hydrogen band EMIC waves (up to 25 nT p‐p) appeared for over 4 h at both Van Allen Probes, from late morning through local noon, when these spacecraft were outside the plasmapause, with densities ~5–20 cm−3. Waves were also observed by ground‐based induction magnetometers in Antarctica (near dawn), Finland (near local noon), Russia (in the afternoon), and in Canada (from dusk to midnight). Ten passes of NOAA‐POES and METOP satellites near the northern foot point of the Van Allen Probes observed 30–80 keV subauroral proton precipitation, often over extended L shell ranges; other passes identified a narrow L shell region of precipitation over Canada. Observations of relativistic electrons by the Van Allen Probes showed that the fluxes of more field‐aligned and more energetic radiation belt electrons were reduced in response to both the emission over Canada and the more spatially extended emission associated with the compression, confirming the effectiveness of EMIC‐induced loss processes for this event.

Azimuthal flow bursts in the inner plasma sheet and possible connection with SAPS and plasma sheet earthward flow bursts

AbstractWe have combined radar observations and auroral images obtained during the Poker Flat Incoherent Scatter Radar Ion Neutral Observations in the Thermosphere campaign to show the common occurrence of westward moving, localized auroral brightenings near the auroral equatorward boundary and to show their association with azimuthally moving flow bursts near or within the subauroral polarization stream (SAPS) region. These results indicate that the SAPS region, rather than consisting of relatively stable proton precipitation and westward flows, can have rapidly varying flows, with speeds varying from ~100 m/s to ~1 km/s in just a few minutes. The auroral brightenings are associated with bursts of weak electron precipitation that move westward with the westward flow bursts and extend into the SAPS region. Additionally, our observations show evidence that the azimuthally moving flow bursts often connect to earthward (equatorward in the ionosphere) plasma sheet flow bursts. This indicates that rather than stopping or bouncing, some flow bursts turn azimuthally after reaching the inner plasma sheet and lead to the bursts of strong azimuthal flow. Evidence is also seen for a general guiding of the flow bursts by the large‐scale convection pattern, flow bursts within the duskside convection being azimuthally turned to the west, and those within the dawn cell being turned toward the east. The possibility that the SAPS region flow structures considered here may be connected to localized flow enhancements from the polar cap that cross the nightside auroral poleward boundary and lead to flow bursts within the plasma sheet warrants further consideration.

Electron precipitation from EMIC waves: A case study from 31 May 2013

AbstractOn 31 May 2013 several rising tone electromagnetic ion cyclotron (EMIC) waves with intervals of pulsations of diminishing periods were observed in the magnetic local time afternoon and evening sectors during the onset of a moderate/large geomagnetic storm. The waves were sequentially observed in Finland, Antarctica, and western Canada. Coincident electron precipitation by a network of ground‐based Antarctic Arctic Radiation‐belt Dynamic Deposition VLF Atmospheric Research Konsortia and riometer instruments, as well as the Polar‐orbiting Operational Environmental Satellite (POES) electron telescopes, was also observed. At the same time, POES detected 30–80 keV proton precipitation drifting westward at locations that were consistent with the ground‐based observations, indicating substorm injection. Through detailed modeling of the combination of ground and satellite observations, the characteristics of the EMIC‐induced electron precipitation were identified as latitudinal width of 2–3° or ΔL = 1 Re, longitudinal width ~50° or 3 h magnetic local time, lower cutoff energy 280 keV, typical flux 1 × 104 el cm−2 sr−1 s−1 > 300 keV. The lower cutoff energy of the most clearly defined EMIC rising tone in this study confirms the identification of a class of EMIC‐induced precipitation events with unexpectedly low‐energy cutoffs of <400 keV.

Contribution of proton and electron precipitation to the observed electron concentration in October–November 2003 and September 2005

Abstract. Understanding the altitude distribution of particle precipitation forcing is vital for the assessment of its atmospheric and climate impacts. However, the proportion of electron and proton forcing around the mesopause region during solar proton events is not always clear due to uncertainties in satellite-based flux observations. Here we use electron concentration observations of the European Incoherent Scatter Scientific Association (EISCAT) incoherent scatter radars located at Tromsø (69.58° N, 19.23° E) to investigate the contribution of proton and electron precipitation to the changes taking place during two solar proton events. The EISCAT measurements are compared to the results from the Sodankylä Ion and Neutral Chemistry Model (SIC). The proton ionization rates are calculated by two different methods – a simple energy deposition calculation and the Atmospheric Ionization Model Osnabrück (AIMOS v1.2), the latter providing also the electron ionization rates. Our results show that in general the combination of AIMOS and SIC is able to reproduce the observed electron concentration within ± 50% when both electron and proton forcing is included. Electron contribution is dominant above 90 km, and can contribute significantly also in the upper mesosphere especially during low or moderate proton forcing. In the case of strong proton forcing, the AIMOS electron ionization rates seem to suffer from proton contamination of satellite-based flux data. This leads to overestimation of modelled electron concentrations by up to 90% between 75–90 km and up to 100–150% at 70–75 km. Above 90 km, the model bias varies significantly between the events. Although we cannot completely rule out EISCAT data issues, the difference is most likely a result of the spatio-temporal fine structure of electron precipitation during individual events that cannot be fully captured by sparse in situ flux (point) measurements, nor by the statistical AIMOS model which is based upon these observations.

Proton and alpha particle precipitation onto the upper atmosphere of Venus

We study the precipitation of protons and alpha-particles onto the upper atmosphere of Venus, using particle data recorded by the Venus Express spacecraft inside the induced magnetosphere. Our investigations are limited to the dayside close to the terminator. We observe on average a net downward flux of protons, which originate partly from the planetary atmosphere and partly from the solar wind. We present median energy spectra of the precipitating protons divided into two energy ranges, 10–100eV and 100eV–30keV. The total dayside precipitation of solar wind protons is estimated to be 3×1022s−1, assuming only protons with energies above 500eV will reach the exobase. Downgoing protons are frequently observed but only in 3% of the available data records we see He2+. These observations are made close to the induced magnetosphere boundary and we argue that at lower altitude the countrates for alpha-particles fall below detection limits. We estimate the precipitation of He2+ onto the dayside exobase to be 1×1021s−1, which is not enough enough to replace the helium escaping from the planet.

Some characteristics of the magnetospheric source of dayside subaroural proton precipitations during magnetospheric compression

Some peculiarities of the source region of proton precipitations and electromagnetic ion-cyclotron waves on the dayside during magnetospheric compression are considered. Flashes of proton emission observed by the IMAGE satellite in the dayside sector equatorwards from the proton aurora oval are used to localize this region. The data from the LANL geostationary satellites, the projections of which during magnetospheric compression were within the proton emission flash, made it possible to find that the source region of the proton precipitations is usually located outside the plasmasphere. In periods of increased geomagnetic activity, this region is located closer to the Earth. Using NOAA satellite data, it is shown that in the dayside outer magnetosphere, precipitations of energetic protons with relatively low intensity are observed prior to magnetospheric compression. This fact shows that the conditions for the development of ion-cyclotron instability are fulfilled there. Magnetospheric compression leads to a sharp increase in the instability increment and, as a consequence, to a sharp growth in the fluxes of precipitating protons, exceeding the level needed for the registration of proton auroras on board the IMAGE satellite.