## 1 Introduction  
Modern-day fusion experiments aim to achieve good plasma confinement, but this is a challenge due to the limitations that different magnetohydrodynamic (MHD) activity places on the accessible operating regimes [1]. Certain MHD instabilities can modify the magnetic geometry of a magnetic confinement device and thus potentially lead to a reduction in plasma confinement and loss of stored energy. In addition, experimental evidence suggests that disruptions are connected to MHD activity. Different MHD modes have been observed to influence plasma in different ways [2]. In order to achieve good plasma confinement, effort must be made to avoid or control these instabilities. Accordingly, it is vital to research and understand the reasons for their appearance in plasma. For this purpose, an analysis must be undertaken of the spatial structure and time behavior of MHD instabilities, or a comparison of their mode structure with theory. Significant work has been carried out to study and understand one particular type of MHD instability, the tearing mode (TM) [3, 4]. Interest in the TM concerns its detrimental consequences for tokamak performance. For one, the TM leads to a non-symmetric plasma equilibrium, which can eventually cause a disruption to occur. The TM can be described as a series of magnetic islands that enhance the radial particle and energy flux from the center in the regions of its formation. This, in turn, degrades plasma confinement and reduces central plasma temperature. Additionally, as the pressure is constant on flux surfaces, the formed island flux surfaces flatten the pressure profile and reduce the central pressure, which is detrimental to tokamak performance. Another issue associated with the TM is the eddy currents it induces, which can potentially cause a loss of global plasma rotation and stability. Research into the tearing mode has been undertaken on a multitude of magnetic confinement devices, such as tokamaks or stellarators, and a variety of tools have been used to study MHD instability. The diagnostics used allow researchers to measure the temperature, density, current profiles and other plasma parameters, to observe the effect of the TM on plasma performance. These include soft X-ray cameras (SXR), electron cyclotron emission (ECE) and the motional Stark effect (MSE). Some of the methods can reveal information regarding MHD instability. For example, magnetic measurements are made using Mirnov coils or magnetic probes. Additionally, microwave techniques, such as conventional reflectometry or the Doppler backscattering (DBS) method, allow researchers to quantitatively describe density fluctuations that include coherent MHD modes. This type of MHD activity was observed and investigated on the previous Globus-M tokamak [5; 6]. Tearing modes with mode numbers of \(m/n=2/1\) were found to develop during the plateau phase of the plasma current of the discharges. Similar MHD activity has been detected on the upgraded spherical Globus-M2 tokamak, which is characterized by a two-fold increase in the toroidal magnetic field up to 1 T (compared to Globus-M). Several types of tearing modes with unique characteristics have been detected under different discharge conditions. In this paper, we present the results of an investigation of those TMs using the DBS method, which has previously been employed to study a variety of oscillatory processes on the Globus-M2 tokamak [7; 8; 9; 10; 11]. The paper is structured as follows. First, the tearing modes that did not lead to degradation of plasma confinement are discussed. The conditions for their development are presented, together with observations made using magnetic coils. This is followed by a brief description of the DBS method, along with confirmation that DBS measurements can provide information regarding TMs. Then, DBS data are presented, which include the localization of the TM and an estimate of its island width. After that, we outline the results of our investigation into TMs with a notable impact on the plasma performance.