# Typical wavelength of solar flare

I read that solar flares are customarily viewed in H-alpha light, as a temporary brightening of a small portion of chromosphere.

What all can be interpreted from this? Is it because, energy of the radiation contained by the flare lies around this wavelength? And why chromosphere ?

• H-alpha is deep red visible light (656.28 nm). Solar flares are classified (A,B,C,M,X) according to total energy output between 0.1 and 0.8 nanometers unc.edu/~rowlett/units/scales/solar_flares.htm That's hard X-rays. Dec 7 '14 at 23:57

Solar flares are observed at wavelengths right across the electromagnetic spectrum, not just H alpha.

The basic model for a solar flare starts with the magnetic field in the corona. You can think of the topology of the magnetic field to consist of loops that poke up out of the photosphere and extend into the corona. However, the photosphere of the Sun is turbulent and constantly in motion due to convection and differential rotation. Whilst a loop may be formed in a minimum energy state, it can get twisted and stressed by these motions.

At some point an instability is reached and the magnetic field can undergo a "reconnection" event, to flip back to a lower energy configuration. During this event, charged particles are accelerated and travel down the magnetic field lines towards the photosphere.

Before they get there, they encounter the chromosphere, which is where the bulk of the particle kinetic energy is deposited. i.e. the density increases as you go down towards the photosphere and once a certain column density is reached, the accelerated electrons are stopped and deposit their kinetic energy. This results in heating and excess H alpha emission from material at about 10 thousand kelvin in the flare footpoints. Any hotter than this and all the hydrogen is ionised. The H alpha is in emission because the only material above it is optically thin to the H alpha radiation. There is hotter, ionised material produced too, and much of this is evaporated such that it fills the magnetic loops with X-ray emitting plasma at temperatures of more than a million kelvin. Some of the flare energy may also be used to accelerate material away from the Sun in "coronal mass ejections".

In the transition from a higher electron energy level to a lower one, say $m\mapsto n$, a hydrogen atom emits a photon of wavelength $\lambda$ satisfying $$\frac{1}{\lambda} = R_\infty\left[\frac{1}{n^2}-\frac{1}{m^2}\right]\text{,}$$ where $R_\infty = 1.09737315685\,\mathrm{m}^{-1}$ is the Rydberg constant. For $n=1$, i.e. the destination energy level is the ground state, varying $m$ forms the Lyman series: $\text{Ly}_\alpha$ ($2\mapsto 1$), $\text{Ly}_\beta$ ($3\mapsto 1$), $\text{Ly}_\gamma$ ($4\mapsto 1$), etc. The $n=2$ destination energy level forms the Balmer series: $\text{Ba}_\alpha$ ($3\mapsto 2$), $\text{Ba}_\beta$ ($5\mapsto 2$), etc., which was actually the first series to be discovered, and is frequently labeled simply with hydrogen instead.

What all can be interpreted from this? Is it because, energy of the radiation contained by the flare lies around this wavelength? And why chromosphere ?

A solar flare is a very hot, violent event that radiates energy across the electromagnetic spectrum. The importance of the H-α line is due to the conveniences of observation.

The spectral lines of hydrogen are outside the visible band except for the first four of the Balmer series, from the red H-α line to the violet H-δ line. When an hydrogen ion and an electron recombine into an atom, the result is generally a hydrogen atom in an excited state. Eventually, it decays to the ground state, but it doesn't have to transition directly there, and typically does so in a random sequence of transitions. A very sizable fraction of those transitions, however, include the $3\mapsto 2$ jump that produces the H-α line.

Thus, the presence of the H-α line is an easy way to identify ionized hydrogen, and in particular, a sudden brightening of the H-α line in an emission-line spectrum is an indicator that something energetic is happening to ionize the hydrogen (moreso than usual, that is). And that's where the chromosphere, the low-density "atmosphere" surrounding the Sun, comes in: it has an emission-line spectrum, i.e., its spectrum is bright in narrow bands that correspond to its atomic or molecular composition. This is unlike the photosphere, which has an absorption-line spectrum instead.

• Thanks for the answer. But why is that photosphere has an absorption line spectrum while chromosphere has emission line spectrum? Dec 7 '14 at 16:28