# RMCSat: An F10.7 Solar Flux Index CubeSat Mission
## 1 Introduction  
Microwave solar emissions at a wavelength of 10.7 cm, or 2800 MHz, are an excellent indicator of solar activity. Measurements of the solar flux density at this wavelength are known as the 10.7 cm solar radio flux, or the F10.7 index. This index is widely used in thermosphere and ionosphere models because these microwave emissions correlate well with other solar emissions such as extreme ultraviolet (EUV), ultraviolet (UV), and X-rays, which originate from similar regions of the solar atmosphere, namely the upper chromosphere and base of the corona. The correlation with EUV is of particular interest as solar EUV flux is the main energy source in the terrestrial atmosphere producing ionization, disassociation, excitation, and heating [1, 2, 3]. Variability in the ionization levels in the ionosphere affects the propagation of radio waves, which in turn can cause errors in terrestrial and satellite communication and navigation signals [4]. The heating of the neutral atmosphere in the thermosphere causes drag forces to increase on Low Earth Orbit (LEO) objects, disrupting the orbital paths of satellites [5, 6, 7]. Consequently, the monitoring of the EUV flux is important for thermosphere and ionosphere studies and space weather predictions. The most notable benefit of microwave solar emissions is that they can reach the surface of the Earth, while EUV wavelengths are absorbed by the atmosphere, as shown in Figure 1. The atmospheric window for 10 cm microwaves has been critical for F10.7 observations historically. It has also made the measurement of F10.7 solar flux exceptionally robust because it can be reliably and accurately recorded from the ground in all weather conditions with few gaps or calibration issues [8]. F10.7 measurements were therefore used as a proxy for EUV measurements, which were unable to be directly measured until the dawn of space-based platforms in the late 20 th century. While EUV flux can now be monitored from space platforms, the highly reliable and repeatable calibration of the F10.7 solar radio flux measurement, along with its continuous historical record, has maintained its standing as one of the most widely used space weather indices [1; 10; 11]. It is a key input parameter to atmospheric models and provides climatology of solar activity over six solar cycles [6; 8; 12]. This duration is only surpassed by sunspot records [13]. Observations of F10.7 solar flux were initially recorded by radio telescopes based near Ottawa, Canada from 14 February 1947 to 31 May 1991 [1]. As of 1 June 1991, the Dominion Radio Astrophysical Observatory (DRAO) in Penticton, Canada is the only ground-based observatory to collect daily measurements. Two radio telescopes onsite collect three one-hour-long solar measurements per day during daylight hours [1]. While measurements have been historically reliable, the inherent vulnerability of a single-station measurement represents a potential point of failure in global ionospheric and thermospheric modeling [14]. Furthermore, F10.7 is currently under-sampled, with the three daylight measurements from a single site constrained further by the effects of local terrain on usable solar elevation angles. The solar flux output is known to vary over a range of timescales, from intense bursts at millisecond to minute scales, to smaller variations over hours to days, following an overarching 11-year solar cycle [1]. The received 10.7 cm flux of interest will vary over the course of a day as active regions rotate across the surface of the Sun. The magnitude of these variations is often more than a solar flux unit and increases with the level of solar activity. Discrete measurements obtained from a single geographic location over a three-hour total observation period would not necessarily capture these variations and therefore not provide certainty of the average solar flux value.