ANGEOAnnales GeophysicaeANGEOAnn. Geophys.1432-0576Copernicus PublicationsGöttingen, Germany10.5194/angeo-35-475-2017Relation of anomalous F region radar echoes in the high-latitude ionosphere to auroral precipitationDahlgrenHannahannad@kth.sehttps://orcid.org/0000-0001-5596-346XSchlatterNicola M.https://orcid.org/0000-0001-6802-1842IvchenkoNickolayhttps://orcid.org/0000-0003-2422-5426RothLorenzhttps://orcid.org/0000-0003-0554-4691KarlssonAlexanderSchool of Electrical Engineering, Royal Institute of Technology KTH, Stockholm, SwedenSchool of Physics and Astronomy, University of Southampton, Southampton, UKHanna Dahlgren (hannad@kth.se)22March201735347547920January201728February2017This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://angeo.copernicus.org/articles/35/475/2017/angeo-35-475-2017.htmlThe full text article is available as a PDF file from https://angeo.copernicus.org/articles/35/475/2017/angeo-35-475-2017.pdf
Non-thermal echoes in
incoherent scatter radar observations are occasionally seen in the
high-latitude ionosphere. Such anomalous echoes are a manifestation of plasma
instabilities on spatial scales matching the radar wavelength. Here we
investigate the occurrence of a class of spatially localized anomalous echoes
with an enhanced zero Doppler frequency feature and their relation to auroral
particle precipitation. The ionization profile of the E region is used to
parametrize the precipitation, with nmE and hmE being the E region peak
electron density and the altitude of the peak, respectively. We find the
occurrence rate of the echoes to generally increase with nmE and decrease
with hmE, thereby indicating a correlation between the echoes and high-energy
flux precipitation of particles with a high characteristic energy. The
highest occurrence rate of > 20 % is found for hmE = 109 km and
nmE = 1011.9 m-3, averaged over the radar observation volume.
Ionosphere (auroral ionosphere; particle precipitation; plasma waves and instabilities)Introduction
Incoherent scatter radars use thermal fluctuations of electrostatic waves in
the ionosphere plasma to derive plasma parameters. Radar echoes above the
thermal level are a manifestation of electrostatic instabilities
. Evidence for strong Langmuir
turbulence SLT; e.g. has been found in
anomalous radar echoes in the form of enhanced
zero Doppler frequency features in the ion line spectra and enhanced
backscatter at the plasma frequency.
Common to observations of SLT radar signatures is the altitude from which the
anomalous echoes arise, typically 200 to 300 km, and their narrow altitude
extent of a few kilometres to tens of kilometres .
and studied the occurrence of the
anomalous echoes by identifying the ion line signature of the echoes in a
large dataset recorded with the European Incoherent Scatter Scientific
Association (EISCAT) Svalbard radar (ESR). They found that the anomalous
signatures frequently occurred for magnetically disturbed conditions and with
a peak occurrence at around 21 MLT. found the highest
occurrence, as measured over the radar beam width, to be 0.6 % at 21 MLT
and for a local K index of 5. Very little data were available for times with
a K index above 6, giving insignificant statistics. A correlation of the
echoes with high-energy electron precipitation was indicated by the local
time distribution and E region electron density profiles derived from the
incoherent scatter measurements .
The occurrence rates reported by suggest that anomalous
echoes resembling SLT signatures are frequently observed with the ESR in the
disturbed evening sector. Simultaneous magnetometer measurements indicate
magnetic disturbances and, thus, possible particle precipitation on a large
spatial scale. However, the magnetometer measurements cannot be unambiguously
related to the specific volume covered by the radar measurements. It has not
been investigated how often the anomalous echoes appear in regions of
particle precipitation. Furthermore, the question arises under which
conditions the echoes are observed and whether a specific type of
precipitation can be identified as the source of the instability leading to
these radar echoes and possible SLT.
Here we investigate the relation of the SLT-like anomalous radar echoes to
auroral precipitation. We study the E region peak density and peak altitude
for a large number of events where anomalous radar echoes were detected to
gain insight into prevailing conditions and the origin of the echoes.
Data
During the International Polar Year (IPY; 2007–2008), the ESR conducted
nearly continuous observations. The radar was observing in the direction of
magnetic zenith, and ion line data were taken with a range resolution of
4.75 km and a radar integration period, further referred to as a dump, of 6 s
(for more details see, e.g., ). Data from the nearby
located Longyearbyen magnetometer (LYR) are used to evaluate geomagnetic
conditions.
A manual selection of a data subset was conducted from the complete IPY
dataset for this detailed study. Two complete days (1 October 2007 and
21 January 2008) with a high occurrence of the anomalous echoes are analysed.
In addition, twelve 3 h long intervals were selected in the afternoon
magnetic local time sector from other days when the local K index was above
5. These data intervals were expected to contain the anomalous echoes based
on the statistical findings of the automated search of ,
and were still limited enough, rendering themselves useful for the careful
manual analysis as described below. A list of the investigated data intervals
is given in Table .
Investigated data intervals and events in 6 s resolution data.
Start time UTLength (h)Events1 Oct 2007 00:002412121 Jan 2008 00:0024687 Mar 2007 15:0035516 Mar 2007 21:003342 Apr 2007 21:003010 Apr 2007 15:003029 Apr 2007 18:003027 May 2007 15:00304 Jul 2007 19:003022 Sep 2007 18:0033429 Sep 2007 19:0033530 Sep 2007 18:003434 Oct 2007 17:003335 Jan 2008 19:003384426Analysis and results
A manual search was conducted for anomalous echoes with enhanced backscatter
power at the zero Doppler shift (following the characteristics of the class
of events reported by and ). The
events were identified in a two-step selection. The first step is based on detecting radar return power enhancements that are narrow in range. Figure shows an example from a
period of 3 min on 1 October 2007. The variation in backscattered power at
E region altitudes (between 100 and 150 km) is the result of ionization by
particle precipitation. At an altitude of 210 km intermittent enhancements
in backscattered power are observed from a thin altitude layer indicating
non-thermal radar backscatter. Enhancements of at least 20 % over the
neighbouring gates were selected (the selected time intervals in the 3 min
interval shown in Fig. are marked with black bars at the top of
the figure) and analysed in the second step based on the shape of the ion
line spectra. To classify as an event for this study, the spectrum for a
given range–time interval was required to have an enhancement of both
shoulders of the ion lines as well as a distinct peak with a zero Doppler
shift. An example of such an ion spectrum can be seen in Fig. 3 in
. The measured increase in power of the echoes ranges
from the threshold of 1.2 up to 30 times the background level. The
enhancements may be even larger, as the echoes are likely more localized than
the spatial and temporal resolution of the IPY experiment.
Received backscattered power as a function of time and altitude on
1 October 2007. In addition to the E region ionization, strong backscatter
enhancements are found in a thin layer at ∼ 210 km altitude. The data
dumps with detected anomalous echoes are marked with black bars at the top of
the figure.
Anomalous echoes were identified in 426 dumps, each corresponding to a 6 s
period, in the following referred to as events. Table gives a
list of the number of events in each investigated data period. For comparison, 87
events were identified in the same data by automatic identification
, which used much more conservative criteria.
Distribution of E region peak altitude and peak electron density for
(a) all investigated data and (b) events. In
(c) the event distribution normalized by the data distribution is
shown. The counts are also given in each bin.
Electron density profiles obtained during the events indicate energetic
electron precipitation, manifested by E region ionization. While the
derivation of the F region parameters from the radar data is affected by the
anomalous echoes, the E region parameters are not, and so reliable density
profiles can still be obtained in the E region. The height of the E region
electron density peak, hmE, is a measure of the characteristic energy of
particle precipitation, and the peak electron density, nmE, is related to the
energy flux. The event distribution as a function of hmE and nmE therefore
gives insight into the characteristics of the electron precipitation during
the anomalous radar echoes.
To retrieve the electron density altitude profiles from the radar
observations, radar data were integrated over three dumps, 18 s, and
standard analysis was applied . The E region electron peak
density and peak altitude were then extracted from these profiles. The
observations were binned by hmE and nmE to investigate the occurrence of the
anomalous echoes for different conditions.
Figure a shows the distribution of all data points binned
by hmE and nmE, with the number of dumps shown inside each of the bins.
Figure b shows the distribution of the identified events
over the same bins. The majority of the events occur for
hmE = 100–145 km and nmE = 1011–1012 m-3.
Figure c shows the event distribution normalized by the
data coverage. Bins with fewer than three events are not shown to avoid the
points with poor statistics. After the normalization, the bins with a high
anomalous echo occurrence rate are concentrated in the parameter space region
with high peak electron densities and low altitudes of the peak,
characteristic of high-energy, high-flux precipitation. The peak occurrence
rate is for the bin centred at nmE = 1011.9 m-3 and
hmE = 109.1 km with anomalous echoes in 20 % of the measurements.
Discussion and conclusions
We have found that the SLT-like anomalous radar echoes from the F region
around 200–300 km altitude frequently occur during high-energy flux
precipitation of particles with a high characteristic energy. The hmE and nmE
values for which the occurrence maximizes are characteristic for electron
precipitation with an energy flux of the order of 10 mW m-2 and a peak
energy of about 3–10 keV e.g.. However,
the exact relation between nmE and hmE and flux and energy depends on the
profile of the atmospheric density and composition (atomic to molecular
ratio), which varies with season, time of day and geomagnetic activity.
Moreover, in the auroral region empirical models exhibit large deviation for
individual events e.g..
This study is complementary to that of . While they
applied an automated routine on a very large dataset to obtain the
large-scale occurrence of the events as a function of magnetic local time and
activity level, here we study the relation of the mechanism behind the plasma
instability to auroral precipitation, detecting a strong connection between
the two. In the large dataset study, it is important to keep the number of
false detections down, hence the strategy of applying a high threshold and
manually validating the automatically detected events. This gives reliable,
but conservative results. In this study of the subset of the data, we can
ensure positive detection of all the events, including somewhat weaker ones,
which nonetheless exhibit the same basic characteristics (localized in
altitude ion line enhancement with a central feature). This indicates that
the plasma instability leading to this class of enhancements is rather common
during energetic auroral precipitation.
The anomalous radar echoes discussed in this study are different from
classical naturally enhanced ion acoustic lines (NEIALs) previously discussed
by, e.g., , , and
. NEIALs are observed as enhancement of ion line
shoulders over a wide altitude range (several hundred kilometres), with
several order-of-magnitude increases of one or both ion line shoulders – and
no features with a zero Doppler shift. While NEIALs have been found to be
related to soft precipitation, both statistically and on an
event basis e.g., the SLT-like anomalous echoes in
this study were found to be closely related to energetic precipitation,
characteristic of discrete aurora. This is in accordance with the event study
of a similar zero Doppler shift feature seen during high-energy precipitation
discussed by . However,
modelling studies
point to the low energy part of the electron
spectrum as being efficient in exciting the Langmuir turbulence. The
electrostatic instability may be coupled to energy-degraded and secondary
electron populations, rather than the primary precipitation. This may be due
to either observational selectivity or physical mechanisms at play.
There are several observational constraints imposed by the data used. First
of all, the radar observes only one value of the wave vector spectrum of the
plasma density disturbance. The instability may cover a larger volume in
space and/or a larger extent in the wave vector space than is readily
observable by the radar. Second, the radar observations average over the
illuminated volume and the integration time, thus underestimating
the scattering
enhancement if it is localized or intermittent. High-resolution observations
with synthetic aperture interferometry would be needed to resolve this
see, e.g.,, but to our knowledge such
observations have not yet been reported for the type of enhancement discussed
here. Finally, the geometric effect of the radar beam covering a larger area
in the F region compared to the E region means that the appearance of the
location of the precipitation (guided by the magnetic field, with much
smaller divergence between the E and the F regions) in the two regions may
differ: the regions at the edge of the radar beam in the F region map outside
the beam in the E region and may feature different precipitation from that
derived from the observed E region profile. This effect will smear out the
statistical dependence in cases when auroral precipitation is structured in
space. In particular it may explain the cases when events appear on the edges
of auroral precipitation (as reported in, e.g., ).
It is not yet resolved what drives the SLT in the ionosphere, but the
observations discussed here indicate a correlation between SLT and electron
precipitation with an energy of at least a few kilo-electronvolts.
Radar data for the IPY 2007–2008 are available through the EISCAT data portal, at
https://www.eiscat.se/schedule/schedule.cgi. The magnetometer data can be obtained through the Tromsø Geophysical Observatory data portal, at
http://flux.phys.uit.no/geomag.html.
The authors declare that they have no conflict of
interest.
Acknowledgements
Hanna Dahlgren was supported by the Swedish Research Council under grant
350-2012-6591. Lorenz Roth was supported by VINNOVA grant 2014-01459 and the
Göran Gustafsson foundation. EISCAT is an international association
supported by research organizations in China (CRIRP), Finland (SA), Japan
(NIPR and STEL), Norway (NFR), Sweden (VR), and the United Kingdom (NERC).
Operation of the EISCAT Svalbard radar during the IPY was supported by the
Norwegian Research Council (NFR), through the IPY-ICESTAR project 176045/S30,
and the USA (NSF). Data from the Longyearbyen magnetometer were provided by
the Tromsø Geophysical Observatory, Norway.
The topical editor, S.
Milan, thanks one anonymous referee for help in evaluating this paper.
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