Abstract
We report magnetic field observations of the components transverse to the main field in the frequency range 1-25 Hz from times of equatorial plasma irregularity crossings. These field variations are interpreted as Alfvenic signatures accompanying intermediate-scale (150 m – 4 km) plasma density depletions. Data utilized are the high-resolution CHAMP magnetic field measurements sampled at 50 Hz along the north-south satellite track. The recorded signals do not reflect the temporal variation but the spatial distribution of Alfvenic signatures. This is the first comprehensive study of Alfvenic signatures related to equatorial plasma bubbles that covers the whole solar cycle from 2000 to 2010. A detailed picture of the wave characteristics can be drawn due to the large number (almost 9000) of events considered. Some important findings are: Alfvenic features are a common feature of intermediate-scale plasma structures. The zonal and meridional magnetic components are generally well correlated suggesting skewed current sheets. The sheets have an orientation that is on average deflect by about 32° away from magnetic east towards upward or downward depending on the hemisphere. We have estimated the Poynting flux flowing into the E region. Typical values are distributed over the range 10-8 - 10-6 W/m2. Large Poynting fluxes are related to steep spectra of the Alfvenic signal, which imply passages through regularly varying electron density structures. No dependence of the Poynting flux level on solar activity has been found. But below a certain solar flux value (F10.7 < 100 sfu) practically no events are detected. There is a clear tendency that large Poynting flux events occur preferably at early hours after sunset (e.g. 20:00 local time). Towards later times the occurrence peak shifts successively towards lower energy levels. Finally we compare our observations with the recently published results of the high-resolution 3-D model simulations by Dao et al. (2013).
Highlights
During the hours after sunset frequently ionospheric plasma irregularities occur at equatorial and low latitudes
In this paper we have presented the characteristics of Alfvénic wave structures associated with intermediate-scale plasma density irregularities at equatorial regions
We have focused on the observational facts of the phenomena and could not give convincing explanations for all of the presented effects
Summary
During the hours after sunset frequently ionospheric plasma irregularities occur at equatorial and low latitudes. In addition there are fluctuations of the transverse magnetic components observed These are interpreted as signatures of Alfvén waves (e.g., [2, 8, 9]) associated with field-aligned currents. Such Alfvén waves are transient features with a lifetime of several tens of seconds (see [1]). An often quoted mechanism is a sudden disruption of zonal electric currents by the rising plasma bubble This effect launches Alfvén waves, which are carried by field-aligned currents (FACs) flowing within the walls of the bubble (e.g., [9]). Active regions in terms of Alfvén waves are smaller plasma depletion structures, which seem to experience repeated triggering within the Alfvén wave travel time (between F and E region) or wave decay time
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