On the relation of Langmuir turbulence radar signatures to auroral conditions

Abstract

We present a statistical study of anomalous radar echoes observed in the auroral ionosphere thought to be signatures of Langmuir turbulence (LT). Data obtained with the European Incoherent Scatter Svalbard radar during the international polar year (IPY) were searched for these anomalous echoes in the auroral F region. In incoherent scatter radar experiments LT may in certain circumstances be observed as enhanced backscattered radar power at the ion line frequencies, plasma line frequencies, and at zero Doppler shift. The power enhancement at zero Doppler shift could arise due to Bragg scattering from nonpropagating density fluctuations caused by strong LT. In the IPY data set, around 0.02% of the data comply with our search criteria for altitudes above 190 km based on the ion line spectrum including enhancement at zero Doppler shift. The occurrence frequency of the identified events peaks in the premidnight sector and increases with local geomagnetic disturbance. Enhanced backscattered power is observed with limited altitude extent (below 20 km in 70% of the events), and the altitude distribution of identified radar signatures in the ion line channel has a peak at about 220 km. Enhancement of the plasma line is observed with the ion line enhancements in more than 60% of the events. Two classes of enhanced plasma lines occur. In the first class, plasma lines are limited in frequency and altitude and occur at altitudes of ion line enhancements. In the second class, the plasma lines are spread in frequency and range and are observed at lower altitudes than the first class (at about 170 km) with frequencies close to 3 MHz. Available optical data available indicate that the identified events to occur during auroral breakup with high‐energy electron precipitation.

Relation of anomalous F region radar echoes in the high-latitude ionosphere to auroral precipitation

“…They found that the anomalous signatures frequently occurred for magnetically disturbed conditions and with a peak occurrence at around 21 MLT. Schlatter et al (2014) 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.…”

“…The occurrence rates reported by Schlatter et al (2014) 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.…”

“…Ekeberg et al (2012) and Schlatter et al (2014) 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.…”

“…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 (Schlatter et al, 2014).…”

Plasma line observations from the EISCAT Svalbard Radar during the International Polar Year

“…We have not focused here on strongly enhanced plasma lines, related to strong Langmuir turbulence (e.g. Schlatter et al, 2014), which are a topic of separate study.…”