the “Little Ice Age.” Such an experiment would resolve the conflict among more indirect measures of the history of the solar constant, some indicating a significant brightening of the Sun since Galileo’s time and some claiming no brightening.
The information about the variation of the solar constant, derivable from the Moon’s regolith temperature profile, has important implications for the interpretation of Earth’s currently observed global warming and is a prerequisite to any estimate of humanity’s contribution to global warming.
Radio observations of solar eruptions have played an important role in understanding the Sun, but the terrestrial ionosphere blocks all radio frequencies below 10 MHz to 20 MHz, which cover virtually all radio emissions originating above 1 to 2 solar radii from the Sun’s surface.
Multi-site observations of low-frequency radio signals from the Moon, where there is no blocking ionosphere, would allow imaging of the sites of particle acceleration in the extended solar corona. In this region, the primary radio sources are fast (2 keV to 20 keV) electrons from solar flares and suprathermal electrons (~100 eV) accelerated by shocks. The associated radio emissions are called Type III bursts and Type II bursts, respectively. Both sources produce a plasma instability, which leads to amplification of electrostatic waves, some of which are then converted to electromagnetic (radio) waves. The process takes place at the characteristic frequency of the plasma—the electron plasma frequency—thus, the frequency of the radio emission indicates directly the electron density of the source, and imaging the radio source would map the extent of the acceleration region.
To make such images at low frequencies would require a synthetic aperture that is large compared with the wavelengths of interest. An angular resolution of 1 degree at 1 MHz requires a minimum diameter of 15 km. The Moon offers a large, stable surface on which to build a large, capable low-frequency radio array for the purpose of imaging solar sources at wavelengths that cannot be observed from the ground, an array that is well beyond the current state of the art for antennas in space. Figure 6.1 illustrates an antenna concept, operating at somewhat higher frequencies for radioastronomy purposes, being deployed on the Moon.
The design for the first (test) array would be based on the designs of Earth-based arrays (working at higher frequencies) currently in development. The deployment of the initial test elements could be done by astronauts or robotic deployers or both. Subsequent deployment of additional elements of the array would be carried out over time, permitting one to maximize lessons learned from implementing each phase.
The major components of the observatory are the antenna array and electric connections, the radio receivers, the central processing unit, the high-gain antenna unit, and the power unit. For a test array with 16 to 32 antennas, the expected total mass for a lunar observatory would be significantly less than 400 kg, and the power requirement would be less than 500 W.
Were it visible to the naked eye, Earth’s magnetosphere would be the most spectacular object in the lunar sky. Subtending about 20 degrees at Full Earth and gradually expanding to envelop the entire lunar sky near New Earth, the magnetosphere would be an awe-inspiring phenomenon. Elements of the magnetosphere can, however, be imaged from the Moon through the use of modern instrumentation on timescales and at spatial resolutions of great interest to magnetospheric plasma physicists and specialists in space weather analysis and forecasting.
A unique advantage of the lunar perspective would be the ability to stare at the space neighborhood of Earth, providing global long-term images of the time development of extended magnetospheric phenomena such as the plasmasphere and ring current. The time period for significant evolution of these phenomena is a few hours, a time interval over which the perspective of an Earth-orbiting satellite changes significantly, while observations from the Moon do not vary appreciably in perspective.