University of Oulu

Validation of neutron monitor data for studies of cosmic ray modulation

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Author: Väisänen, Pauli1,2
Organizations: 1University of Oulu Graduate School
2University of Oulu, Faculty of Science, Physics, Space physics and astronomy (SpaceAstro)
Format: ebook
Version: published version
Access: open
Online Access: PDF Full Text (PDF, 3.4 MB)
Persistent link:
Language: English
Published: Oulu : University of Oulu, 2023
Publish Date: 2023-06-05
Thesis type: Doctoral Dissertation
Defence Note: Academic Dissertation to be presented, with the assent of the Graduate School of the University of Oulu, for public discussion in the Auditorium L2, Linnanmaa, on June 30th, 2023, at 12 o’clock noon
Tutor: Professor Ilya Usoskin
Professor Kalevi Mursula
Reviewer: Professor Emilia Kilpua
Associate Professor John Clem
Opponent: Doctor Monica Laurenza
Kustos: Professor Ilya Usoskin


The near-Earth space and upper atmosphere experience a continuous barrage of particles from different sources: The magnetized solar wind flow from the Sun and the highly energetic cosmic rays originating from (extra-)galactic sources or solar eruptions. Energetic cosmic rays can also create particle air showers that reach the ground level at Earth.

The flux of cosmic rays arriving at Earth can be measured at different locations by employing neutron monitors (NMs). We can use NM data to study the local variation of cosmic radiation or use a combination of NMs with knowledge about the geomagnetic shielding to model the flux of cosmic rays in near-Earth space.

The flux of galactic cosmic rays (GCRs) arriving to the heliosphere, the region of solar plasma influence, is believed to be constant on millenial time scales. Therefore, the variations of cosmic rays we observe inside the heliosphere are related solely on the heliospheric modulation of cosmic rays through deflection in magnetic fields. This modulation is driven by solar activity and is highly periodic. The 11-year solar cycle is the strongest modulator, but also periods of about 1.75 years, 155 days and the solar rotational 27 days can be identified. Also the local anisotropy of cosmic rays at Earth causes a daily variation in CR measurements.

Since the heliospheric medium is highly turbulent, magnetic irregularities generated by it cause modulation of GCRs. By studying power spectrum slopes of NM variations, we found evidence that the level of turbulence affecting NM variations is also dependent on the solar cycle phase.

Over one hundred NMs have been active since the 1950s. The NM data is available from multiple databases, but it was found that these datasets are not uniform with one another. A subsequent survey of 147 NM stations from over 300 data records analysed the situation and made it possible to construct a recommended data source list for users of NM data.

With the improved knowledge of NM station data quality, we used data from multiple NM stations and new NM yield function estimations to construct an updated version of the heliospheric modulation potential, which is a measure of the average energy loss of GCR particles in the heliosphere at 1 AU. This new method is highly scalable and accurate and was used to produce a daily version of the modulation potential reconstruction.

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Osajulkaisut / Original papers

Osajulkaisut eivät sisälly väitöskirjan elektroniseen versioon. / Original papers are not included in the electronic version of the dissertation.

  1. Väisänen, P., Usoskin, I., & Mursula, K. (2017). Structure of the power spectral density of galactic cosmic ray variation during 1953-2016. Proceedings of the International Astronomical Union, 13(S335), 82–86.

    Rinnakkaistallennettu versio / Self-archived version

  2. Väisänen, P., Usoskin, I., & Mursula, K. (2019). Long‐term and solar cycle variation of galactic cosmic rays: Evidence for variable heliospheric turbulence. Journal of Geophysical Research: Space Physics, 124(2), 804–811.

    Rinnakkaistallennettu versio / Self-archived version

  3. Väisänen, P., Usoskin, I., & Mursula, K. (2021). Seven decades of neutron monitors (1951–2019): Overview and evaluation of data sources. Journal of Geophysical Research: Space Physics, 126(5), e2020JA028941.

    Rinnakkaistallennettu versio / Self-archived version

  4. Väisänen, P., Usoskin, I., Kähkönen, R., Koldobskiy, S., & Mursula, K. (2023). Revised reconstruction of the heliospheric modulation potential for 1964–2022. Journal of Geophysical Research: Space Physics, 128(4), e2023JA031352.

    Rinnakkaistallennettu versio / Self-archived version

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Series: Report series in physical sciences
ISSN: 1239-4327
ISSN-E: 2814-967X
ISSN-L: 1239-4327
ISBN: 978-952-62-3732-9
ISBN Print: 978-952-62-3731-2
Issue: 154
Type of Publication: G5 Doctoral dissertation (articles)
Field of Science: 115 Astronomy and space science
Funding: I am grateful for the financial support by Academy of Finland ReSOLvE Centre of Excellence and project ESPERA and by the Finnish Cultural Foundation.
Copyright information: © University of Oulu, 2023. This publication is copyrighted. You may download, display and print it for your own personal use. Commercial use is prohibited.