On the relationship between auroral absorption, electrojet currents and plasma convection

Abstract

Abstract. In this study, the relationship between auroral absorption, electrojet currents, and ionospheric plasma convection velocity is investigated using a series of new methods where temporal correlations are calculated and analysed for different events and MLT sectors. We employ cosmic noise absorption (CNA) observations obtained by the Imaging Riometer for Ionospheric Studies (IRIS) system in Kilpisjärvi, Finland, plasma convection measurements by the European Incoherent Scatter (EISCAT) radar, and estimates of the electrojet currents derived from the Tromsø magnetometer data. The IRIS absorption and EISCAT plasma convection measurements are used as a proxy for the particle precipitation component of the Hall conductance and ionospheric electric field, respectively. It is shown that the electrojet currents are affected by both enhanced conductance and electric field but with the relative importance of these two factors varying with magnetic local time (MLT). The correlation between the current and electric field (absorption) is the highest at 12:00-15:00 MLT (00:00-03:00 MLT). It is demonstrated that the electric-field-dominant region is asymmetric with respect to magnetic-noon-midnight meridian extending from 09:00 to 21:00 MLT. This may be related to the recently reported absence of mirror-symmetry between the effects of positive and negative IMF B y on the high-latitude plasma convection pattern. The conductivity-dominant region is somewhat wider than previously thought extending from 21:00 to 09:00 MLT with correlation slowly declining from midnight towards the morning, which is interpreted as being in part due to highenergy electron clouds gradually depleting and drifting from midnight towards the morning sector. The conductivitydominant region is further investigated using the extensive Correspondence to: R. A. Makarevich (r.makarevich@latrobe.edu.au) IRIS riometer and Tromsø magnetometer datasets with results showing a distinct seasonal dependence. The region of high current-absorption correlation extends from 21:00 to 06:00 MLT near both equinoxes, however, it is narrower and rotated towards the morning (02:00-07:00 MLT) in summer, while in winter the correlation shows much greater variability with MLT. During periods of high current-electricfield correlation, the relationship between electric field and absorption can be described as an inverse proportionality, which can be explained by limitation of the electrojet current by the magnetospheric generator. Possible cases of electron heating absorption are also investigated with absorption showing no obvious dependence on the ion velocity or electron temperature.

Polar mesosphere summer echoes with ESRAD, Kiruna, Sweden: Variations and trends over 1997–2008

“…This is close to the correlation coefficient of 0.5-0.7 which we find between daily mean values of the Kiruna K index and CNA measured by the riometer in Abisko for the years 2000-2004 (not presented here). Kellerman et al (2009) showed also that correlation between magnetic perturbation and ionospheric electric field is higher than that between the former and CNA. This might imply that the K and Ap indices are more likely associated with ionospheric electric field than with electron density variations below 100 km.…”

“…In turn, CNA is determined by (heightintegrated) electron density mainly below 100 km. Recently Kellerman et al (2009) reported that the horizontal component of magnetic perturbations in the auroral region is highly correlated with CNA only during the 3 h after magnetic local midnight (which is about 1 h before solar midnight in Kiruna). At other times the correlation is poor, resulting in a daily averaged value of 0.4 (our estimation).…”

Polar mesosphere summer echo strength in relation to solar variability and geomagnetic activity during 1997–2009

“…We add to this table the results from Fig. 6 of Kellerman et al (2009) for the correlation between perturbations in the horizontal component of the geomagnetic field (δH ), which is the basis of the K-index, cosmic noise absorption (A) representing conductance due to energetic particle precipitation, and ion convection velocity (V i ) as a proxy for ionospheric electric field. From 18:00 UT to 06:00 UT, when the correlation between δH and A is high (>0.5), it is reasonable to suggest that electron density variations due to energetic particle precipitations may be the cause of those in PMSE η.…”

“…These disturbances are caused by those in ionospheric electrojet currents which depend on both conductance (heightintegrated conductivity) and hence electron density, and ionospheric electric field. Kellerman et al (2009) showed that effect of conductance prevails during night-time and that of electric field dominates during daytime. In Table 3 we present correlation coefficients between η and K-index for each 3-h time interval together with probability (p) that there is no correlation between them.…”

“…NLC occurrences from ground-based Fig. 6a of Kellerman et al (2009) and UT time was recalculated from magnetic local time (MLT) time for Kiruna as UT = MLT − 2.5 h. p is probability that there is no correlation between η and K-index.…”