Skywave modes of radio communication operate by bending ( refracting ) radio waves ( electromagnetic radiation ) through the Ionosphere . During the "peaks" of the solar cycle, the ionosphere becomes increasingly ionized by solar photons and cosmic rays . This affects the propagation of the radio wave in complex ways that can either facilitate or hinder communications. Forecasting of skywave modes is of considerable interest to commercial marine and aircraft communications , amateur radio operators and shortwave broadcasters . These users occupy frequencies within the High Frequency or 'HF' radio spectrum that are most affected by these solar and ionospheric variances. Changes in solar output affect the maximum usable frequency , a limit on the highest frequency usable for communications.
The explanation of fluctuations in aggregate economic activity is one of the primary concerns of macroeconomics . The main framework for explaining such fluctuations is Keynesian economics . In the Keynesian view, business cycles reflect the possibility that the economy may reach short-run equilibrium at levels below or above full employment . If the economy is operating with less than full employment, ., with high unemployment , Keynesian theory states that monetary policy and fiscal policy can have a positive role to play in smoothing the fluctuations of the business cycle.