Energetic electron precipitation during sawtooth injections

Abstract

Abstract. We present simultaneous riometer observations of cosmic noise absorption in the nightside and dawn-noon sectors during sawtooth particle injections during 18 April 2002. Energetic electron precipitation (>30 keV) is a feature of magnetospheric substorms and cosmic radio noise absorption acts as a proxy for qualitatively measuring this precipitation. This event provides an opportunity to compare the absorption that accompanies periodic electron injections with the accepted paradigm of substorm-related absorption. We consider whether the absorption is consistent with the premise that these injections are quasi-periodic substorms and study the effects of sustained activity on the level of precipitation. Four consecutive electron injection events have been identified from the LANL (Los Alamos National Laboratory) geosynchronous data; the first two showing that additional activity can occur within the 2-4 h sawtooth periodicity. The first three events have accompanying absorption on the nightside that demonstrate good agreement with the expected pattern of substorm-absorption: discrete spike events with poleward motion at the onset followed by equatorward moving structures and more diffuse absorption, correlated with optical observations. Dayside absorption is linked to gradient-curvature drifting electrons observed at geostationary orbit and it is shown that low fluxes can lead to a lack of absorption as precipitation is suppressed; precipitation begins when the drifting electron flux surpasses some critical level following continuous injections of electrons from the magnetotail. In addition it is shown that the apparent motion of absorption determined from an azimuthal chain of riometers exhibits an acceleration that may be indicative of an energisation of the drifting electron population.

High-latitude geomagnetically induced current events observed on very low frequency radio wave receiver systems

“…At high geomagnetic latitudes, geomagnetic substorms can be observed as increases in the absorption levels of riometers as a result of energetic electron precipitation in the 30–200 keV range [ Kavanagh et al , 2007]. As we have previously shown (Figure 5) that the NBE occurrence frequency peaks at midnight in the same way that substorm signatures do, we might anticipate an association between riometer observations and the NBEs.…”

Signal absorption effects on HF radio paths near Sodankyla observatory (Finland)

“…It is known that precipitation resulting in absorp tion can considerably change in space (Hargreaves et al, 2010). This depends on the substorm injection posi tion (Kavanagh et al, 2007) and on the presence of such discrete energetic forms as an omega band (Kavanagh et al, 2007) or geomagnetic pulsations modulating precipitation (Beharrel, 2010). In our case Sodankyla observatory is located at a rectilinear dis tance of ~300 km from the GRK-LOZ path.…”

Observations of omega bands using an imaging riometer

“…It is immediately clear that a number of electron injections and field dipolarizations occurred throughout the day; these are indicative of a series of substorms onsets that repeat with a periodicity of ∼2.2 h. This is suggestive of a Sawtooth event (e.g. Borovsky et al, 2001;Henderson et al, 2002;Kavanagh et al, 2007). Thus the observation of traditional substorm-like characteristics (i.e.…”