Geosci. Instrum. Method. Data Syst. Discuss., https://doi.org/10.5194/gi-2017-53 Manuscript under review for journal Geosci. Instrum. Method. Data Syst. Discussion started: 4 December 2017 c Author(s) 2017. CC BY 4.0 License. Investigation of a low-cost magneto-inductive magnetometer for space science applications Leonardo H. Regoli1, Mark B. Moldwin1, Matthew Pellioni1, Bret Bronner2, Kelsey Hite1, Arie Sheinker3, and Brandon M. Ponder4 1Climate and Space Sciences and Engineering, College of Engineering, University of Michigan, Ann Arbor, USA 2Space Physics Research Laboratory, College of Engineering, University of Michigan, Ann Arbor, USA 3Magnetic Sensing, Soreq Nuclear Research Center, Israel 4Nissan Technical Center North America (NTCNA), USA Correspondence to: Leonardo H. Regoli ([email protected]) Abstract. A new sensor for measuring low-amplitude magnetic fields that is ideal for small spacecraft is presented. The novel measurement principle enables the fabrication of a low-cost sensor with low power consumption and with measuring capabilities that are comparable to recent developments for CubeSat applications. The current magnetometer, a software- modified version of a commercial sensor, is capable of detecting fields with amplitudes as low as 8.7 nT at 40 Hz and 2.7 nT 5 at 1 Hz, with a noise floor of 500 pT/√Hz @ 1 Hz. The sensor has a linear response to less than 3% over a range of 100,000 nT . All of these features make the magneto-inductive principle a promising technology for the development of ± magnetic sensors for both space-borne and ground-based applications to study geomagnetic activity. 1 Introduction Magnetic fields are a ubiquitous feature of our solar system and of key importance for geophysical, magnetospheric and 10 heliospheric investigations. The sun produces the interplanetary magnetic field (IMF) and many of the planets and moons throughout the solar system produce their own magnetic fields through dynamo and magneto-inductive response processes. Even where no internally produced magnetic field is present, for example, Mars, or Venus, the IMF plays a major role in how planets and smaller bodies interact with the solar wind. At Earth, the measured field is a combination of the internal dynamo-generated field and perturbations that occur in space, 15 particularly during substorm and geomagnetic storm processes. These processes are governed by the direction of the IMF and the dynamic pressure exerted by the solar wind at any given time (e.g., Moldwin, 2008). The enhancement of the particle fluxes in the ring current during a geomagnetic storm causes the measured magnetic field strength at the surface of the Earth to decrease. This is quantified by the so-called disturbance storm time (Dst) index, which is determined by a network of low-latitude magnetometers (e.g., Hamilton et al., 1988; Liemohn et al., 2001). 20 The dynamic nature of planetary magnetospheres makes it extremely difficult, if not impossible, to understand their structure without the help of a magnetometer with sufficient resolution, dynamic range, and bandwidth, to discriminate between the different regions inside the magnetosphere and identify the magnetic signature of plasma flows that are governed by global and 1 Geosci. Instrum. Method. Data Syst. Discuss., https://doi.org/10.5194/gi-2017-53 Manuscript under review for journal Geosci. Instrum. Method. Data Syst. Discussion started: 4 December 2017 c Author(s) 2017. CC BY 4.0 License. local circulation patterns. For this reason, magnetometers have been a key tool in magnetospheric investigations throughout the history of their study and continue to be indispensable. Critically, current and planned investigations of multi-scale dynamic features throughout the solar system continue to drive the need for greater numbers of magnetometers with state of the art capabilities. 5 1.1 ULF waves in the magnetosphere The Earth’s magnetosphere, whose field strength varies from about 60,000 nT in polar LEO orbit to about 100 nT at geosyn- chronous orbit, have different wave populations present with frequencies ranging from a few mHz to a few Hz on both the day- and nightside. Traditionally, the continuous pulsations which are denoted by Pc1-5 can be divided into categories that are characterized by a given frequency range as summarized in Table 1 (e.g., Jacobs et al., 1964; Fraser, 2007; Menk, 2011). Table 1. ULF waves in the magnetosphere. Wave Frequency Pc1 0.2 5 Hz − Pc2 0.1 0.2 Hz − Pc3 22 100 mHz − Pc4 7 22 mHz − Pc5 1 7 mHz − 10 These waves provide an insight into magnetospheric dynamics including wave-particle interactions (mostly Pc1 and Pc2), the solar wind activity (Pc3 to Pc5, Takahashi et al. 1984, Takahashi and Ukhorskiy 2008), as well as internal processes (e.g., Hartinger et al., 2014). Several studies have focused on the relationship between fluctuations in solar wind conditions and the observation of ULF waves in the magnetosphere and on ground stations. Among others, Kessel (2008) used data from the ACE, Wind, Geotail, 15 Cluster and GOES satellites and from ground stations to perform a statistical study during a period of time of over a month and found that for most of the time when Pc5 waves were observed, their amplitude and power were related to fluctuations in the solar wind, with only about 20% of the total power coming from internal processes. A similar dependence on solar wind conditions has been observed for waves in the Pc3 (e.g., Constantinescu et al., 2007; Clausen et al., 2009) and the Pc3-4 range (Heilig et al., 2007). 20 Given their dependence on different aspects of the interaction between the solar wind and the magnetosphere, Pc3 to Pc5 waves, when measured on the dayside, provide a way of studying how the global magnetosphere reacts to changes in helio- spheric parameters such as solar wind density, solar wind speed and IMF (e.g., Shen et al., 2015; De Lauretis et al., 2016; Takahashi et al., 2016; Shen et al., 2017). The use of ground-based magnetometers, depending on their distribution around the globe and in combination with global 25 models of the magnetosphere, can also shed light on how global the distrubances are, by correlating the signals observed at different latitudes with the length of the corresponding magnetic field lines. 2 Geosci. Instrum. Method. Data Syst. Discuss., https://doi.org/10.5194/gi-2017-53 Manuscript under review for journal Geosci. Instrum. Method. Data Syst. Discussion started: 4 December 2017 c Author(s) 2017. CC BY 4.0 License. In addition, and due to the dependence of the Alfvén velocity on the local plasma denstiy, ground magnetometers can be used to infer low-energy populations that are difficult to measure in space due to spacecraft charging effects (Menk et al., 1999). In a similar manner, the observation of field line resonances has been used to infer other properties of the magnetosphere such as location (Dent et al., 2006), density (Berube et al., 2003) and composition (Takahashi et al., 2008) of the plasmapause or also 5 the location of the open-closed field line boundary (Ables and Fraser, 2005). One of the difficulties of studying waves in the magnetosphere is that conditions change rapidly and thus standing waves are difficult to maintain (Kivelson, 2006). This translates into a strong damping of the waves and thus multi-point observations are necessary to study the different regions affected at the same time. The use of small satellites with commercial off-the-shelf (COTS) instruments on board opens the possibility of having large, 10 cost-effective constellations, making it possible to study both large structures that are visible at magnetosphere-scales as well as small structures such as magnetic reconnection that are close to the electron scales (Burch et al., 2016). 1.2 Measurement approaches With the growing interest of the scientific community in small satellites as a tool to perform magnetospheric and heliospheric studies, the need for space instruments that are cheaper and easier to produce has given rise to the study of different possibilities 15 including the use of COTS components or complete instruments. Originally CubeSats were mostly seen as technology demon- stration platforms, however there are now missions with scientific instrumentation being flown and proposed (e.g., Moretto, 2008; Springmann et al., 2012; Klesh et al., 2013; Heine et al., 2015; Lepri et al., 2017; Goel et al., 2017; Zurbuchen et al., 2016). When it comes to magnetometers, due to their reliability, performance and ability to measure low fields, two types of 20 sensors have predominantly been used for space missions, namely fluxgate and helium magnetometers. However, due to their high fabrication costs, relatively large size and high power needs, different alternatives have been recently studied for CubeSat missions. One approach is to miniaturize fluxgate magnetometers, while the other is to explore chip-based COTS technologies such as magneto-resistive and Hall magnetometers. Miles et al. (2016) developed a miniature fluxgate magnetometer with noise floor of about 200 pT/√Hz @ 1 Hz with a 25 power consumption of 400 mW , considerably smaller than instruments used in large missions such as Cassini (up to 12.63 W combining a fluxgate and a vector helium magnetometer, Dougherty et al. 2004) and, more recently, the Magnetospheric Multiscale Mission (MMS, almost 2 W for the fluxgate instrument, Russell et al. 2016). Also using the fluxgate measurement principle, Matandirotya et al. (2013) compared three COTS instruments with special focus on parameters relevant for space applications. They identified one specific magnetometer, the LEMI-011B that, after 30 modifications involving separation of the sensor and the electronics, was able to achieve noise levels below 1 nT @ 12.83 Hz and 30 mW power consumption. Taking advantage of the mobile phone popularization that happened during the last decade, Ponder et al.