Comets can be compared to dirty snowballs. We do not know much about comets with dimensions below 10m because they are difficult to observe. But it is possible that small comets supply water to the upper atmosphere as they evaporate.
Meteors are small pebbles which light up as shooting stars when they impact the upper atmosphere. Most are small and burn up in the atmosphere, but some reach the earth's surface. Little is known about meteors of dimensions smaller than 1m.
ASIM will observe the precipitation of meteors and small comets and their effect on the atmosphere. The measurements will give new insights into the formation of the Solar System and how the atmosphere is affected by space.
Solar particle and UV radiation affecting NOx concentrations
NO and NO2 are often referred to as NOx. In the stratosphere, where the protecting ozone layer is formed, NOx destroys ozone. It is important then to understand how NOx is formed in the upper atmosphere. One of the sources of NOx is solar UV-radiation which forms NOx from oxygen and nitrogen. Another source is energetic particles ejected in violent eruptions on the sun and traveling through space to bombard the earth's magnetosphere and atmosphere. Both sources of NOx are high in the atmosphere. The NOx formed here is transported downwards affecting ozone concentrations in the stratosphere.
ASIM måler NO direkte, samtidigt med målinger af indfald af energetisk partikelstråling - gennem dets excitation af atmosfæren (nordlys) og udstråling af røntgenstråling. Med ASIM kan vi for første gang undersøge direkte dannelsen af NOX samtidigt med vi kender den energetiske partikelstråling.
The interplay of the ionosphere, magnetosphere and thunderstorms
Normal lightning is made up of electrical discharges carrying high currents between different regions of a thundercloud or between thunderclouds and the ground. Giant lightning high above thunderstorms, on the other hand, carrys currents between the clouds and the ionosphere. In both cases, lightning currents act as large antennas that emit a short but powerful electromagnetic signal. This impulse can propagate to the ionosphere causing severe heating and excitation and leading to optical emissions known as Elves. The heating of the lower edge of the ionosphere above thunderstorms can lead to absorption and drop-out of radio transmitter signals, which propagate in the earth-ionosphere wave guide through the affected region.
The pulse can travel further through the ionosphere and into the magnetosphere, where interactions with energetic electrons cause these to be dumped into the atmosphere. In this process, the ionosphere experiences heating and enhanced ionization.
The perturbation to the ionosphere degrades the precision of GPS systems. With the ASIM instruments, both the source of the perturbations (lightning and energetic particle radiation) and changes in GPS signal characteristics measured by other instruments on the International Space Stations are measured, allowing accurate modeling of the effects.
Auroras
Streams of plasma are continuously ejected from the sun into space. This plasma makes up the solar wind. The solar wind interacts with the magnetosphere, compressing it on the front towards the sun and extending the magnetic into a tail directed away from the sun. When particularly violent eruptions eject large amounts of energetic plasma into space, when the particles reach the magnetosphere they compress it, creating showers of energetic particles precipitating into the atmosphere. When they hit the atmosphere, the atmospheric constituents are excited, emitting photons of different colours. This is an aurora.
An aurora is usually created at relatively high magnetic latitudes. However, during large solar storms, auroras move to lower latitudes where they come into reach of the ASIM instruments on the International Space Station.
There are still unanswered questions about auroras and how auroras affects the upper atmosphere. ASIM will be able to study the energetics of auroras both looking horizontally towards the limb, and downwards as the ISS passes over the aurora.