No abstract

Cosmic noise absorption (CNA) measurred by imaging riometer, is an excellent tool to passively study the high latitude D-region ionospheric conditions and dynamics. An imaging riometer has been installed at Indian Antarctic station Maitri (geographic 70.75 • S, 11.75 • E; corrected geomagnetic 63.11 • S, 53.59 • E) in February 2010. This is the first paper using the imaging riometer data from Maitri. The present paper introduces the details of this facility, including its instrumentation, related CNA theory and its applications. Sidereal shift of around 2 hours in the diurnal pattern validates the data obtained from the newly installed instrument. Moreover, the strength of cosmic noise signal on quiet days also varies with months. This is apparently due to solar ionization of D-region ionosphere causing enhanced electron density where collision frequency is already high. The main objective of installing the imaging riometer at Maitri is to study magneotspheric-ionospheric coupling during substorm processes. In the current study, we present two typical examples of disturbed time CNA associated with storm-time and non-storm time substorm. Results reveal that CNA is more pronounced during storm-time substorm as compared to nonstorm-time substorm. The level of CNA strongly depends upon the strengthening of convectional electric field and the duration of southward turning of interplanetary magnetic field before the substorm onset.

On 02 April 2011, a couple of cosmic noise absorption (CNA) events were detected at Maitri, Antarctica (L = 5; CGM 63.14°S, 53.69°E) confining to nighttime and daytime. One of the two events that occurred during night hours was caused due to auroral substorm onset. The current study focuses on the later CNA event, which was recorded during daytime (10:00–13:00 magnetic local time (MLT), MLT = UT‐1, at Maitri, Antarctica). We refer to this CNA event as dayside CNA (DCNA) event. Absence of westward electrojet during DCNA confirms its dissimilarity from auroral substorm absorption events. A comparison has been made between the DCNA event of 02 April 2011 with that of 14 July 2011, a day with substorm activity when Maitri is in dayside but without DCNA event. The comparison has been made in the light of interplanetary conditions, imaging riometer data, ground magnetic signatures, GOES electron flux density, and associated pulsations. The study shows that stronger prolonged eastward interplanetary electric field favors the occurrence of DCNA event. It is concluded that DCNA event is due to the gradient curvature drift of trapped nonrelativistic electrons in the equatorial plane. Estimated energy of trapped electrons using azimuthal drift time for a set of ground stations within the auroral oval confirms the enhancement in electron fluxes in the same energy band as recorded by geostationary satellites GOES 13 and GOES 15. The reason for precipitation of electrons is expected to be the loss cone scattering caused by wave‐particle interaction triggered by ULF waves.

The present study has focused on the intense production of cosmic noise absorption (CNA) at Maitri, Antarctica (L = 5; CGM −62°S, 55°E) during the early recovery phase of the largest storm of the current solar cycle commenced on 17 March 2015 St. Patrick's Day. The enhancement of CNA during 15–18 UT(14–17 magnetic local time (MLT); MLT = UT − 1 at Maitri) was as large as the CNA enhancement occurred during the main phase of the storm. During this time the CNA pattern also exhibits oscillation in the Pc5 (2–7 mHz) range and is in simultaneity with geomagnetic pulsations in the same frequency range. We observed the amplitude of CNA pulsation is well correlated with the level of CNA production. High‐amplitude Pc5 oscillations were observed in the vicinity of auroral oval near Maitri. Absence of electromagnetic ion cyclotron (EMIC) waves is marked suggesting the possible role of VLF waves in precipitation. The reason for the intense CNA production is found to be the precipitation caused mainly by hiss‐driven subrelativistic electrons. The CNA enhancement event is located well inside the dusk plasmaspheric bulge region as suggested by Tsurutani et al. (2015). Signature of enhanced eastward electrojet at Maitri during 14–17 MLT could be an additional factor for such large CNA. In order to establish the cause and effect relationship between the geomagnetic and CNA oscillations at Maitri, transfer entropy method has been used, which confirmed the modulation of CNA by geomagnetic pulsations.

No abstract