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Astron. Astrophys. 354, 714-724 (2000)

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5. Conclusions

The [FORMULA]-line polarisation produced by electron beam impacts with Hydrogen atoms embedded into a flaring atmosphere with magnetic field was calculated using the density matrix approach.

The following conclusions are made:

  1. In absence of electron beams the [FORMULA]-line linear polarisation is determined by the external chromospheric radiation starting from depth [FORMULA] downwards to the photosphere and it reaches a magnitude about [FORMULA].

  2. The [FORMULA]-line profiles show a linear polarisation in a range of [FORMULA] only in the line cores where a `weak magnetic field' approximation appears to be valid; the wings are fully depolarised by thermal collisions.

  3. The circular polarisation is negligible.

  4. On the top of atmosphere linear polarisation in the [FORMULA]-line profile is negative for the transitions with a bigger J being about [FORMULA] and positive for a smaller J reaching about [FORMULA]. From depth of [FORMULA] the polarisation in all transition changes to a negative one being about [FORMULA] at the beginning of beam injection and decreasing to [FORMULA] at maximum of the beam flux.

  5. The full [FORMULA]-line linear polarisation caused by impacts with beam electrons is negative varying from [FORMULA] to [FORMULA] for viewing angles [FORMULA] and [FORMULA], respectively.

  6. The full [FORMULA]-line linear polarisation varies from [FORMULA] in the upper chromosphere for the first seconds of electron beam injection to [FORMULA] in the lower chromosphere for a maximum energy flux of the beam.

  7. There is a secondary maximum of linear polarisation appearing in the lower chromosphere at a column depth of [FORMULA]. This is caused by an increasing abundance of low energy beam electrons precipitating downwards below the upper chromosphere where the return current electric field becomes negligible.

Accordingly, it has been shown that electron beams can be the agents producing some features observed in the [FORMULA]-line polarisation during the early phases (first few minutes) of solar flares. In order to discriminate the effects of electron beams from those caused by proton beams, the complex multi-wavelength observations with high temporal and spectral resolution are required.

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© European Southern Observatory (ESO) 2000

Online publication: February 9, 2000