Traditionally, the effect of solar energetic particles on our atmosphere was seen as a disturbance in the ionisation of the upper atmosphere, leading to e.g. absorption of HF radio waves, as in the case of solar proton precipitation, causing the polar cap absorption phenomenon. However, the precipitating high energy particles have also other important impacts on the atmosphere, such as changes in the chemistry and composition. Although ionisation degree of the atmosphere in general is very low, it turns out that via production and loss of chemically important minor neutral constituents, the ion-chemical reactions can affect even the neutral chemistry. High energies of the particles may lead to direct effects even down to the stratosphere and transport of chemically long-lived constituents may lead to long term effects during sequences of solar particle events. In this paper, research on the effects of solar energetic particles on atmospheric chemistry is first shortly reviewed and then a more detailed treatment is given on the recent work in Finland, concerning quantitative modelling of the local forcing of the upper atmospheric chemistry and the first experimental verifications of this forcing using satellite data. Examples are given on effects of selected solar proton events on odd nitrogen and ozone.

Solar energetic particle events, auroral particle precipitation and relativistic electron precipitation can lead to significant momentary production of odd nitrogen through dissociation, ionisation and subsequent ion-chemical processes. In absence of solar radiation during the polar night, nitric oxide is a long-lived constituent. Atmospheric circulation is expected to transport excess amounts of NO created in the auroral zone to lower latitudes. Molecular diffusion in the thermosphere and eddy diffusion in the lower thermosphere and mesosphere are expected to transport NO to lower altitudes. As the NO descends, it is converted to NO₂ through its reaction with ozone. As there is no back reaction in the polar night, the NO₂ is long lived and continues its journey into the lower atmosphere. Thermospheric NO might be carried down to the stratosphere, where it would enhance the background density of odd nitrogen and participate in the catalytic destruction of ozone.

In previous estimates the role of ionic processes has not been taken into account in detail. We have developed a detailed, coupled ion and neutral chemistry model with 70 unknowns in order to calculate the time development and local response during excess ionisation events. During extreme solar proton events, ionic sources of the neutral nitric oxide are seen to dominate over sources by neutral chemistry. Negative ions are seen to directly destroy ozone. A model study of a representative electron precipitation case, occuring in the midnight sector, was seen to enhance nitric oxide dramatically not only during the event itself. The effect sustained in the timescale of one day even up to next noon so that 4-fold electron densities resulted at altitudes from 80 to 100 km, when compared to a control model run without precipitation. Photochemistry at daytime started to decrease the amount of enhanced nitric oxide but this process was not strong enough to really compete with enhancement prevailing due to the previous night's precipitation.