The magnetic local time (MLT) dependence of electron (0.15–300 keV) and proton (0.15–6900 keV) precipitation into the atmosphere based on National Oceanic and Atmospheric Administration POES and METOP satellite data during 2001–2008 was described. Using modified APEX coordinates the influence of particle energy, substorm activity and geomagnetic disturbance on the MLT flux distribution was statistically analysed.
Some of the findings are the following.
Substorms mostly increase particle precipitation in the night sector by about factor 2–4, but
can also reduce it in the day sector. MLT dependence can be assigned to particles entering the magnetosphere at the cusp
region and magnetospheric particles in combination with energy-specific drifts MLT flux differences of up to 2 orders of magnitude have been identified inside the auroral oval during geomagnetically disturbed conditions. The novelty here is the comprehensive coverage of energy bands and the focus on asymmetry. The maximum flux asymmetry ratio depends on particle energy, decreasing with Kp for low energetic particles and increasing with Kp for higher energy electrons, while high energy protons show a more complex dependency. While some aspects may already have been known, the quantification of the flux asymmetry sheds new light on MLT variation.
Particle precipitation is a primary link between solar activity and atmospheric chemistry.
The interplanetary medium is the driver of geomagnetic disturbance and may compress, deform or reconnect to the magnetosphere.
MLT dependence is a result of charge-dependent drift directions
Substorms are either directly driven or/and loading processes, where energy is accumulated and released abruptly in the Earth's magnetosphere
The occurrence of substorms depends on the orientation of the interplanetary magnetic field. As shown in
In this study we will discuss MLT differences in particle fluxes (and precipitation) over a wide energy range and show how substorms impact this pattern.
In Sect.
This section describes the data sets and how the data have been processed.
For particle data we use time series (2001–2008) of 16 s averaged electron fluxes ranging from 0.15 to 300 keV and protons from 0.15 to 6900 keV measured onboard the polar-orbiting NOAA/POES and their successor, the METOP satellites
In total all available data from POES 15, 16, 17, and 18 and METOP 02 have been used, except for POES 16 after 2006, as it is known that the TED data are erroneous.
All satellites have Sun-synchronous orbits at altitudes around 820 km (with
Information about the different channels can be found in Table
It is known that there is no adequate upper energy threshold of the three MEPED electron channels
Channels and nominal energy ranges from the POES and METOP satellites which have been used.
We used the 0
The MEPED detectors have a field of view of
Given that the point of view of the TED detector is almost identical to the MEPED detector and the field of view is significantly smaller, Fig. 1 in
All figures in this paper show differential particle flux in (MeV m
However, it is known that anisotropic distributions occur. While an unaccelerated source population is assumed to be isotropic (as is a wave-scattered fraction of that population in the loss cone), most acceleration processes are connected with an anisotropic pitch angle distribution.
In case of an anisotropic pitch angle distribution an estimation of the total precipitating flux is not straightforward as first a pitch angle distribution has to be assumed and second the pitch angles the detector is currently measuring have to be determined. Since the only other detector orientation on POES measures trapped particles (at high latitudes) and since trapped particles do not get lost, there is no reason to assume a smooth transition between these two particle populations. Thus we do not have a “reference” anisotropic pitch angle distribution that might be applied. Applying an isotropic pitch angle (which is often done in the literature) will put the downgoing flux on a level with precipitating flux. In the case where the paper states “particle precipitation”, this isotropic pitch angle distribution has been implicitly assumed. However, this has been made without loss of generality since the shown differential flux in that case is equal to the downgoing flux. Thus no transformation is needed.
All shown values are spatially and temporally averaged fluxes. In case a detector measures zero counts every time, it crosses a specific position, and at a certain condition this also enters the figures with zero flux (see e.g. Fig.
The particle detectors suffer from various contamination effects: the MEPED electron channels are highly efficient detectors for high energetic protons. In order to avoid contaminated electron data, we excluded MEPED electrons when the omnidirectional proton channel P7 showed more than two counts (based on high-resolution 2 s data). This cuts out probably contaminated periods not only in SPEs, but also in the region of the South Atlantic Anomaly (SAA). The MEPED electron channels have been subtracted from each other, resulting in differential channels.
Note that the given energy ranges taken from
A meaningful representation of particle precipitation has high requirements for the coordinate system as follows.
The flux pattern should be invariant in time even though the magnetic field is changing (meaning moving poles, not magnetospheric distortion). This is needed for the long investigation period as well as for durability of forecasts. The latitude of the particle flux pattern should be invariant of the longitude. Given this criterion the longitude may be replaced by local time as a second coordinate. If the previous criterion is applied, it includes the fact that particle flux has to be recalculated. Following the footpoints of two shells with a distinct magnetic field strength, their latitudinal distance differs with longitude. Since the particle flux is measured on a fixed detector size, this has to be taken into account when removing the longitudinal dependence. Particle measurements take place at the position of the satellite, which is about 820 km above the ground. But the effect of particle precipitation (the atmospheric ionization) is mainly located at about 110 km altitude (maximum of magnetospheric ionization; higher particle energies cause ionization further down). Consequently a coordinate system that allocates the satellite's measurement to their respective position at 110 km altitude would be helpful.
The coordinate system that allows for all named requirements is the modified APEX coordinate system
The period 2001–2008 was chosen for our investigation, where all necessary data about substorms and particle fluxes are available.
For the identification of substorm events, we use the technique published by
It should be noted that with this technique we are not able to separate different substorm phases nor can we distinguish different types of substorms. Independent of substorm phase, the proton aurora is displaced equatorward of the electron aurora for dusk local times, and it is poleward for dawn local times. In the onset region, however, proton and electron precipitation depends on the substorm phase and may even be co-located
The Kp index is a 3-hourly index estimating the geomagnetic activity
The particle data have been binned into 11 partly overlapping Kp-level groups: 0–0.7, 0–1, 1–1.7, 1.3–2.3, 2–2.7, 3–3.7, 4–4.7, 5–5.7, 6–6.7, 6–9 and 7–9.
As the Kp levels are not equally populated (low Kp levels occur more frequently than e.g. 6–6.7), the number of satellites is not constant, the substorms are not evenly distributed in time and the local time sectors are not evenly covered, single data points (with 1 h MLT resolution, 2
Binning of particle flux strongly depends on the coordinate system. Some features are determined by the inner magnetic field and are thus co-rotating with Earth, while others are influenced by the interaction with the solar wind and according to that fixed in relation to the Sun and to the (magnetic) local time. Since we will use the modified APEX coordinates in this paper, we will have a look at how the particle flux representation differs from geographic coordinates and which aspects can be best described in the two systems.
Figure
Particle flux in TED proton band 11 in geographic and modified APEX 110 km coordinates. The colour scale marks the minimum flux in the auroral oval in beige. The neighbouring colour indicates that the flux is a factor
Most obvious in the geographic representation (Fig
A feature that is connected to the SAA is the particle precipitation of the drift loss cone. Particles drift around Earth and bounce between the mirror points. These mirror points get to the lowest altitudes where the magnetic field is weakest. Since the geomagnetic field around the SAA is weak, the dominating particle precipitation zone is where these field lines have their foot points. In Fig.
Figure
Apart from that the geographic representation is not very helpful. Due to the satellites' inclinations the poles are not covered, and the typical pattern as the auroral precipitation is meandering.
Switching to magnetic modified APEX 110 km coordinates (see Fig.
Most obvious in the modified APEX/MLT coordinates (see Fig.
Some regions in modified APEX/MLT coordinates will never be reached as the local time coverage is limited. This holds for the midnight hours in the Northern Hemisphere as well as for noon in the low-latitudinal Southern Hemisphere.
The equatorial region seems not to be covered, but this is not a data gap. The flux is mapped to the latitude where the guiding field line hits 110 km. Since the satellites cross the (dip) Equator at 850 km, all the field line peaks below that point are not covered (
As a consequence of the regional coverage and SAA influence, we will select the Southern Hemisphere auroral zone for further investigation.
If we would take a look at the footpoints of a solar-synchronous satellite in local time, we would see that it always crosses a particular latitude, e.g. the Equator at one particular local time in ascending mode (and another, at the Equator 12 h later, in descending mode). At high latitudes it crosses 12 local time zones at a few latitudes, but still, the next orbit will exactly match the first (except if the orbit moves, which also happens to the POES/METOP satellites but on longer timescales).
Looking at the footpoints of the same satellite in MLT changes quite a bit. Given that the MLT zones are based on magnetic longitude and the magnetic poles being shifted, it means that the MLT footpoints, especially at high latitudes, differ significantly from one orbit to the next. Due to the POES inclination of 98.5
Thus there are two options for how the MLT during an orbit may develop in the Southern Hemisphere: if the satellite's longitude is far from the magnetic pole, the orbit will not pass the magnetic pole and the MLT will gradually increase by 12 h till it reaches the Equator in ascending mode again. Let us call this “ascending MLT”. In the other case (“descending MLT”), the southern magnetic pole will be passed and the MLT zones will be flown through in the opposite direction, decreasing MLT by 12 h till it reaches the Equator in ascending mode again. Since the southern magnetic pole is somewhat south of Australia, a significant fraction of the orbits passing it will cross the SAA in descending mode (but not in ascending mode). The opposite is true for the ascending MLT path, which includes a significant fraction of orbits that pass the SAA in ascending mode.
In case multiple satellites are used, this does not affect high latitudes, but at low latitudes the situation is different. Since the satellites cross the Equator at two specific local times (for ascending and descending modes, being just slightly broader in MLT), MLT coverage at the Equator is limited to these points. They however may be reached in ascending mode (or left in descending mode) by ascending or descending MLT paths.
In Fig.
The ascending MLT path now connects e.g. the low flux right edge of the 21 MLT equatorial crossing with the high flux (SAA) left edge of the 9 MLT equatorial crossing. The descending MLT path on the other side connects e.g. the high flux (SAA) left edge of the 21 MLT equatorial crossing with the low flux right edge of the 9 MLT equatorial crossing.
In summation, the ascending and descending MLT paths cause the left edge of an equatorial crossing to be affected by the SAA, while the right edge is not. Any MLT analysis of latitudes that show longitudinal variations will suffer from the fact that longitudes contribute very unevenly to the MLT zones in the polar orbit of POES/METOP. Given that the SAA is the dominant flux source at low latitudes, this hampers a MLT flux analysis here. Effects may also be seen in the drift loss cone, where longitudinal flux variations are expected. At high latitudes however, just minor longitudinal variations (in magnetic coordinates) are expected (see Fig. 1, bottom left, auroral zone). Consequently it just has minor affects on the results but not on the overall findings and trends. Additionally this effect gets counterbalanced by broader MLT coverage and multiple satellites at high latitudes.
In some parts of the paper we will refer to the APEX 110 km latitude or MLT locations of the auroral oval or its flux maximum and minimum. These locations have been determined automatically. A routine determines the maximum flux for each MLT bin within the typical auroral latitude range. This results in a preliminary auroral oval. Then the latitudinal differences between MLT predecessor and successor are determined, and in the case of large outliers a point is assumed to be a spike in the data and replaced by the next biggest flux bin in that MLT zone. In case more than seven points have to be replaced for an auroral oval, the corresponding channel-Kp set is neglected. In summation this ends up in a well-working detection algorithm for the auroral oval and allows us to find its minimum and maximum fluxes or their ratio. A sample is given in Fig.
Sample for an auroral oval fit. The grey dots represent the position of the auroral oval. The green (9 MLT) and black (20 MLT) dots indicate the maximum and minimum of the auroral oval, respectively.
Figure
The colour scale is logarithmic with a base of 2, meaning the threshold to the adjacent colour is a factor of 2 apart. The reference value has been set individually for every channel to the lowest occurring value inside the auroral oval. Thus local time differences can be easily identified and quantified. No-substorm periods (left panel) and isolated substorm periods (right panel) for all electron channels are given here.
Figure
Electron flux in various channels at high latitudes in the Southern Hemisphere. The left panel shows periods without substorms and the right panel gives periods with isolated substorms only.
Proton flux in various channels at high latitudes in the Southern Hemisphere.
Comparing the two panels of Figs. a typical pattern in low energetic channels (see Sect. a typical pattern in high energetic channels (see Sect. Kp dependence of the auroral MLT asymmetry (see Sect. auroral oval asymmetry during substorms (see Sect. a latitudinal displacement of the maximum auroral flux depending on Kp and energy (see Sect.
Low energetic proton and electron channels, namely TED electron bands 4 and 8 as well as proton bands 4, 8 and 11, show a very different spatial pattern than the higher channels.
The maximum flux in the auroral oval appears in the day sector. TED electron bands 4 and 8 peak between 6 and 17 MLT.
This agrees e.g. very well with the monoenergetic electron number flux for low solar wind driving
The proton bands are even more concentrated around noon but show an additional slight increase from noon via the morning sector towards midnight. Since this is completely opposite to the higher channels, we will have a look at the source region.
The main precipitation of low energetic electrons (
In contrast, during periods with isolated substorms the particle flux is shifted by 2
In summation our findings confirm that high numbers of low energetic particles enter the magnetosphere preferentially on the front side through cusp and other boundary layers
Additionally, low energetic protons (TED proton bands 4–14) show a second oval structure (approx. 50–65
The high electron channels (above
Concerning protons, between TED proton band 14 and the following channels (mep0P1 to mep0P3) the main particle flux shifts from midnight to the evening sector, which is oppositely directed to the electron displacement.
A potential explanation for the displacement in the higher electron channels (and the oppositely directed shift of the protons) is the westward partial ring current in the night side which is closed by field-aligned currents (Birkeland Region 2) into the ionosphere
The resulting auroral asymmetry also depends on Kp level, as shown in Sect.
Even without Kp dependence Figs.
While e.g. the two lowest TED electron channels (bands 4 and 8) show just minor MLT variations, it varies by more than 1 order of magnitude in the higher particle energies.
The proton flux shows distinct MLT dependence, ranging from just minor variations in TED proton bands 11 and 14 (as well as practically no MLT variation in the highest proton MEPED channels) up to about 1 order of magnitude in the lowest and medium particle energies (TED proton bands 4 and 8, MEPED mep0P1 and mep0P3).
During isolated substorms the maximum local time differences are similar or a factor of 2 higher.
However, we noticed that the MLT asymmetry is not constant over different Kp levels. This section will emphasize the impact of Kp levels using Fig.
The upper panel shows the two lowest electron channels and the lowest proton channel, which all have a declining flux asymmetry with increasing Kp. The 6–6.7 Kp bin is enhanced here, but we should keep in mind that these levels occur rarely and may suffer from bad statistics. A reason for the decline might be that the cusp inflow is not increasing with Kp as the rest of the auroral flux. Thus its relative fraction declines and subsequently decreases the asymmetry.
The middle panel shows all particle channels that have an increasing flux asymmetry with Kp, that is, all remaining electron channels and the proton channels TED band 11 and mep0P1. Given that high geomagnetic disturbance should be linked with enhanced acceleration, scattering and substorm processes increasing asymmetry in the affected regions suggest themselves.
The lowest panel gives the flux asymmetry dependencies of the remaining proton channels that are less distinct. It seems that there is a domain change at about 3.3 Kp since the asymmetry of TED proton band 14 and mep0P2 has a negative correlation below 3.3 and a positive one above. For the channels TED proton band 8 and mep0P3 the relationship is the opposite.
Dependence of the auroral oval asymmetry with Kp.
The two highest energy channels (MEPED mep0P4 and mep0P5) do not show MLT variations as seen in Fig.
In summation, the maximum MLT asymmetry depends on Kp.
For very low energy (proton and electron), it decreases with Kp. For higher electron channels, it increases with Kp. For higher proton channels the Kp dependence is ambiguous, but in general the asymmetry is significantly smaller than in the electron channels.
This section discusses the changes during substorm periods based on Figs.
In general, the particle flux during isolated substorms is similar to no-substorm periods, but is superimposed with substorm-specific night-side (20–2 MLT) particle precipitation, which reflects the substorm electrojet manifestations
The electron and proton flux intensity at the midnight sector outnumbers the no-substorm values at the same place by factor 2 to 4. For mep0P1 to mep0P3 the evening sector is also slightly increased during substorms. Given the flux increase in the midnight sector, the maximum auroral flux during a substorm can mostly be seen around 0 MLT (see Figs.
Additionally, noon-sector electron fluxes decrease during a substorm, which is clearly seen in all the upper channels (from TED electron band 11 to mep0e2–e3). The noon-sector flux decreases most probably because dayside particle precipitation occurs often during northward-oriented IMF, which is not usual for substorms (see Fig.
In contrast to the electron fluxes, the day-sector proton fluxes do not significantly depend on substorm activity (see Fig.
Figure
The auroral asymmetry is shown as a ratio between maximum and minimum auroral oval fluxes. The number above a specific ratio states the MLT where the maximum is detected, while the number below that ratio indicates the MLT of the minimum.
Except for TED electron band 4 (where there is no significant difference between substorm and no-substorm periods), all other channels have an increased auroral oval asymmetry during isolated substorms. The numbers above and below the marked flux ratio indicate the MLT location of the minimum (below) and maximum (above). We can identify that the maximum flux during a substorm shifts to the midnight sector (if not already there in no-substorm periods), e.g. for mep0e1–e2 or TED protons band 4 and 14.
For TED electron bands 4 and 8 (as well as TED proton bands 8 and 11) the substorm enhancement is also seen in the night sector, but it does not overshoot the dayside flux (see Figs.
This agrees with
The asymmetry in both the electron and proton spectra (as well as during no-substorm or substorm periods) shows a local minimum in middle TED channels (TED electron band 8 and proton band 11) as well as a local maximum at higher energies (TED electron band 14 or mep0e1–e2 for electron and mep0P1 or mep0P2 for protons). At even higher energies the asymmetry declines again.
Figure
The modified APEX 110 km latitude of the maximum flux in the auroral oval is shown. Colours indicate specific Kp levels. The left-hand side displays the energy dependence of electrons, the right-hand side the one of the protons.
The figure has been derived by the auroral oval determination method discussed in Sect.
Except for some outliers, most of them belonging to TED proton band 4 during high Kp levels (
However, there is a noticeable latitudinal shift with particle energy even for the particle channels that appear co-located. For 8 out of 11 Kp levels there is an equatorial shift of 2
Every colour graph represents the spectral location of the maximum flux latitude for a certain Kp range. Thus we can infer that increased geomagnetic disturbance (high Kp values) causes a dislocation of up to about 8
Concerning the outliers in TED proton band 4, for low Kp values there is a clear flux maximum at noon, which is located at rather high latitudes (compare Fig.
In summation, there is an equatorial shift of the main precipitation zone with increasing Kp and increasing particle energy, while the latter is primarily due to a shift in MLT and only secondarily due to a latitudinal shift of the auroral oval itself.
In this paper we presented the MLT distribution of energetic particle flux/precipitation into the ionosphere in combination with different substorm activity.
We could identify low energetic particles to predominantly precipitate around local noon, supporting the idea that they enter the magnetosphere through the cusp. During substorms the maximum particle flux is shifted by 2
Higher particle energies show a different behaviour. Electrons (
There is an energy-dependent auroral asymmetry. While the low energetic electrons have just a minor asymmetry, it enhances to more than 1 order of magnitude for the higher electron channels. For low energetic protons the cusp precipitation causes an asymmetry of about an order of magnitude. Above that energy the asymmetry first declines (to factor 2 in TED proton bands 11 and 14) and then enlarges again with MEPED channels 1 to 3 (more than an order of magnitude). For the highest proton channels the asymmetry disappears as these particles are not linked with auroral precipitation. During substorms the maximum flux is similar or a factor 2 higher.
The auroral asymmetry is Kp-dependent. For low energetic particles the asymmetry declines with Kp, probably due to a lack of cusp precipitation during high Kp values, while it increases especially (stronger) for higher electron channels, probably due to increased acceleration and scattering processes. For medium and high energetic protons the development of the asymmetry with Kp is not that distinct; there might be multiple processes involved.
During substorms the no-substorm flux seems to be generally superimposed by substorm-specific night-side particle flux. However, the noon-sector fluxes depend on particle species. For protons they seem to be independent of substorm activity, while for electrons they decrease during a substorm.
Also, we noticed a Kp- and energy-dependent equatorial shift of the main flux latitude.
Upon request, the data used for the publication of this study are available from the authors: Olesya Yakovchuk (oyakovchuk@uos.de) or Jan Maik Wissing (jawissin@uos.de).
The authors worked on all parts of the paper jointly.
The authors declare that they have no conflict of interest.
This article is part of the special issue “Vertical coupling in the atmosphere–ionosphere system”. It is a result of the 7th Vertical Coupling workshop, Potsdam, Germany, 2–6 July 2018.
The authors acknowledge the NOAA National Centers for Environmental Information (
The work is supported by DFG project WI4417/2-1.
This research has been supported by the German Science Foundation (DFG; grant no. WI4417/2-1).This open-access publication was funded by Osnabrück University.
This paper was edited by Elias Roussos and reviewed by two anonymous referees.