Experiments at the edge of space: balloon flights to the stratosphere

Over 1500 balloons are launched every day, from every continent on Earth, to provide forecasting of tropospheric weather. Similar balloons, which can fly to the edge of space (>30 km), can be used for other science projects. Professional scientists, military users, commercial organisations, and interested amateurs, all fly payloads that provide a relatively low-cost means to reach the upper atmosphere. Weather ballooning is perfectly suited to student education and has been carried out for decades by groups of school, college, and university students. Here we report on one such a project. During March/April 2023 a series of balloons were launched from Sodankylä, Finland, in order to study the particle and radiation environment, along with ozone, in the stratosphere. Inexpensive off-the-shelf Geiger-counters were part of a payload flown to investigate how the radiation environment changed over time. Balloon payloads can be tracked with simple and inexpensive radio receivers. Similar projects to the one outlined here should be possible for any school, college, or university that has a reasonably well-equipped workshop, a group of interested and capable students, and a desire to investigate and learn something new about the planet we live on.


Introduction
Weather balloons provide a simple, low-cost means to reach a region of our planet that is particularly difficult to sample routinely.Too low for satellites, and too high for most aeroplanes, the stratosphere exists at the edge of space and covers an altitude range ∼10-50 km.Below the stratosphere is the troposphere (0 to ∼10 km), where most weather phenomena occur.Above the stratosphere is the mesosphere (∼50-80 km) where the temperature falls to a minimum.The stratosphere itself is a region where the temperature increases with increasing altitude, primarily due to the absorption of UV radiation that is potentially harmful to life on the surface.The wellknown 'ozone layer' exists within the stratosphere (see figure 1).
Flying weather balloons to the stratosphere, along with an appropriate payload, is possible for anyone willing to invest a relatively small amount of money, plus some time and effort.None of the work is particularly difficult, relying on welldocumented processes for launch authorisation.Such projects are perfect suited to small groups of students with a willingness to learn and experiment, along with supervisor(s) who can coordinate the overall aims and goals.Possible payloads are only limited by the imagination of those involved and could include off-the-shelf detectors, cameras, environment sensors, etc.
There are many previous reports of student involvement or student-led science projects that utilise weather balloons (e.g.Larson et al (2009), Bancroft et al (2014), Phillips et al (2016)).Other examples of similar projects can be found in the literature.
Below we describe the conception and planning of our own project, the role of the students involved (co-authors of this paper), and present some of our initial findings.Our overall experiment was a combination of professional scientific research, along with a substantial educational component.Regardless of this, similar projects are within the reach of almost anyone, and we intend this summary report on our project to be of use to similar endeavours carried out around the globe.

Background and project aims
Student involvement in scientific endeavour is not new.It assists in student learning and educational development and provides valuable handson experience.Additionally the students themselves typically come up with many new ideas of how to achieve project aims.In a previous paper we documented how an educational student project successfully measured very low frequency radio waves by means of an experiment deployed at Headlands School and Community Science College, Bridlington (Kavanagh et al 2011, Denton et al 2012).In this present report we document and summarise our work on a different research project, and in particular the contribution to the project by undergraduate and graduate students from the University of Colorado, Boulder, USA.In March/April 2023 a series of balloons were launched from near Sodankylä, Finland, located within the Arctic Circle as part of an international research collaboration.The balloons flew from the surface, through the troposphere and into stratosphere, to more than 30 km altitude.The students involved in the project contributed heavily to the work, and provided valuable assistance in terms of payload design, preparation, and integration.Furthermore, they assisted in launching and tracking the balloons, as well as analysis of data once the payloads were recovered.
The overall aim of the project, carried out as a collaboration between three primary institutes, is to 'assess if and how particles and radiation from space affect ozone levels in the stratosphere'.While some of the logistics and portions of the equipment involved might be beyond the reach of some educational institutes, the overall design, launch process, recovery, and data analysis/interpretation can easily be replicated, along with different payloads that may be more or less sophisticated.

Particle and radiation measurements in the stratosphere
The composition of the stratosphere is wellknown and documented.Starting with some of the first scientific balloon flights, the broad daily, seasonal, and annual changes in temperature, density, humidity, wind, etc have also been measured for decades.From the first flights undertaken before 1900, right up to the present day: the history of early balloon observations is both fascinating and not generally well known (Labitzke and van Loon 1999).One of the first discoveries was the Pfotzer-Regener Maximum.This is a peak in radiation found at ∼25 km altitude and attributed to the effects of galactic cosmic rays (GCRs) (Regener and Pfotzer 1935, Pfotzer 1936) (see also Carlson and Watson (2014)).
Over 1500 balloons are flown to the stratosphere every day, and these are primarily aimed at measuring phenomena related to terrestrial weather forecasting.Additional regular 'soundings' are made to measure atmospheric composition, such as the level of ozone (O 3 ) and its variation with altitude (Kivi et al 2007).Balloons remain the primary means by which insitu stratospheric observations are made.
Observations of energetic particles and radiation in the stratosphere are comparatively rare.As highlighted by Berger et al (2021), 'Remarkably, we know more about the radiation environment onboard the International Space Station than we do…the middle atmosphere'.In addition to the effects of GCRs, particles from the solar-wind and magnetosphere also rain down (i.e.'precipitate') into the atmosphere and cause the aurora borealis (northern lights) and aurora australis (southern lights), generally above ∼90 km altitude.It remains uncertain how more energetic particles affect lower atmospheric regions in detail.While some effects of energetic particles on the stratosphere are known (e.g.McPeters and Jackman 1985, Denton et al 2018) our knowledge is far from complete.The current project is aimed at further developing our understanding of the effects of energetic particles (e.g.electrons/ions) and radiation (e.g.x-rays) on the stratosphere.

Project methodology
The experimental part of the project was carried out at the Finnish Meteorological Institute Arctic Space Centre in Finland (67.367 • N, 26.629 • E, 179 m above sea level).This launch site for the balloon payloads is known to have frequent auroral activity, suitable infrastructure to support the launches, and has been flying stratospheric balloons since 1949 (Kivi et al 1999).In March/April 2023, we carried out a total of 11 stratospheric balloon flights from this site.Many different payloads were launched (both our own instruments and 'guest' instruments).
Here we limit discussion to the work carried out with our own detectors, which comprised of (1) a GMC 500+: an energetic particle/radiation detector; (2) a Vaisala RS41 radiosonde: an instrument that measures temperature, pressure, relative humidity, etc and transmits the results back to a radio receiver on the ground (Dirksen et al 2014, Madonna et al 2020); and (3) an EN-SCI ozonesonde: a sophisticated scientific instrument that measures in-situ ozone with high accuracy (Smit and the ASOPOS Panel 2014).
Prior to flight, we wanted to better understand the response of the GMC-500+ detectora commercial 'off-the-shelf' device that would be used to measure particles and radiation in space.This work included assessing the number of counts registered by the device in different orientations with respect to a test source of radiation (see figure 2).Additional work was carried out to understand how the counts changed with respect to temperature (since the temperature in the stratosphere can fall below −50 C) and to determine the clock-drift on the various flight units.This work was carried out in its entirety by one of the students on this project.
Once in Finland, much of the work involved preparing payloads prior to flight, assisting with launch, and then tracking the balloons, and downloading/plotting/analysing the results.This work, particularly the data analysis, was carried out largely by the students involved.Figure 3 shows an example payload in the process of preparation, along with a photograph of the launch on 25th March 2023.Each detector is enclosed in a styrofoam box, with/without a simple heater as required.The heater can be a simple 9 V battery connected to a resistor that emits a small amount of heat (1-2 W) during flight.The balloon and the boxes are connected via strong, light-weight nylon cord and suitable steel clips and shackles.A parachute (6 or 9 feet in diameter depending on payload weight) was used to slow the descent of the payload after the balloon bursts.
Once airborne, the payload can be tracked via the signal sent out by the Vaisala radiosonde.An entire amateur community of radio enthusiasts participate in tracking weather balloon and various web-resources are available that assist in this including flight trajectory predictions, etc.While we used a Vaisala MW41 system to receive data from the balloon, a very simple radio receiver and antenna provide a low-cost alternative to detect the location of the balloon and this has a range of ∼100 km.Data can be received and uploaded to a dedicated 'Sondehub' website (https://sondehub.org) using this methodology (see figure 4    The same website shows the locations of a wide variety of weather balloons across the world. When finally back on the ground it may not be possible to receive the radio signal from the radiosonde.As backup to ensure we can find the payload on landing we used a simple GPS/GSM ('dog tracker') device that contains a prepaid SIM card and this is programmed to send out a position via SMS (see figures 4(b) and (c)).
For our experiments, permission to launch was given by Air Traffic Control Centre (ATTC) Finland.Payloads less than 3 kg are allowed without use of a radio transponder.ATTC Finland has information regarding the predicted balloon trajectory forecast.We also telephoned roughly 10 min before each launch for final permission.Automatic weather balloons are also launched each day at noon and at midnight from Sodankylä and ATTC Finland are aware of these launches.Radio frequencies used are between 401 and 406 MHz.Different countries have different requirements and regulations requiring radio transmission and balloon launches.For comparison, a brief guide to current regulations in the UK can be found at https://ukhas.org.uk/doku.php?id=guides:faq.If launched from the UK, our experiments would need to be modified since radio transmission from balloons is not permitted.In this case data can easily be stored on board and transmitted when the payload reached the ground (e.g. by mobile phone) or the payload would have to be recovered manually.
Of course, the success of any experiment can only be judged on the results and examples are shown in figure 5, including a screenshot from a GoPro camera flown on one of our flights as an additional payload.Also shown are the 'counts' from the GMC 500+ particle/radiation sensor, and the partial pressure of ozone from the EN-SCI ozone detector on 30th March 2023.The profile of counts detected by our GMC 500+ sensor confirms the existence of the Pfotzer-Regener maximum in the stratosphere, with a peak occurring at ∼20 km altitude.As can be seen, there is considerable variability in both the ozone measurements and the particle counts.

Figure 1 .
Figure 1.Schematic showing different regions of the Earth's atmosphere and the variation of temperature and ozone as a function of altitude. (a)).

Figure 4 .
Figure 4. (a) Tracking the payload using the Sondehub website.(b) Radio tracking device used to locate the payload in flight.(c) A GPS/GSM 'dog-tracker' device is used to locate the payload once it has landed.

Figure 5 .
Figure 5. Example result including: (a) looking down on the Earth from close to 30 km altitude; (b) measurements of the variation of ozone with altitude; (c) counts from the GMC 500+ detector made on the 30th March 2023 flight.
Table 1 below contains a list of equipment used in this project and shows typical values for some of the free parameters involved.